“From the One, Only One Proceeds”

The Post-Classical Reception of a Key Principle of Avicenna’s Metaphysics

In: Oriens
Wahid M. Amin Al-Mahdi Institute UK Birmingham

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The separated intellects play a crucial but notoriously controversial role within the Neoplatonic systems of al-Fārābī and Avicenna. While both thinkers provide an array of proofs to support the existence of such immaterial substances, the most enduring of these is based on a metaphysical rule of Avicenna’s metaphysics known as the “rule of one” (qāʿidat al-wāḥid): that from the One, only one proceeds (lā yaṣdur ʿan l-wāḥid illā l-wāḥid). The following paper explores the various ways in which Avicenna defended this principle and traces their reception in the post-classical period, thereby showing how vigorously the question of emanation was debated among scholars of the later medieval period.

The most distinctive and arguably most controversial feature of the Arabic philosophical movement (falsafa) of the 4–5th/10–11th centuries (AH/CE) was its endorsement of the Neoplatonic scheme of emanation. At the hands of its two major representatives, al-Fārābī and Avicenna, the Muslim falāsifa sought to reconcile how a timeless and eternal First Principle could be held to account for the ‘creation’ of a temporal world. Unlike the mutakallimūn for whom temporality and the origination of the world from nothing were evidence of a timeless and eternal Creator, the falāsifa regarded the notion of creation ex nihilo to be philosophically absurd. First, the idea that substances began to exist from prior nothingness seemed patently false; for where there is nothing to begin with, only nothing remains (ex nihil nihil fit). Second, a First Principle that creates bodies composed of atoms and their accidents (as was believed to be the case by the mutakallimūn) suggests that a God who is perfectly One and simple is the direct cause of things that are both multiplied and composed. However, according to Avicenna, the view that the One qua one causes the many qua many is not only logically absurd but theologically unsustainable also, given that if God were to create the many directly He would no longer be the simple reality befitting His nature and thus could not be necessary of existence in Himself, which by Avicenna’s reckoning is the only true hallmark of divinity. In other words, if the First Principle is one and necessary of existence in Itself, then the first effect that proceeds directly from It must be one, encapsulating what has since become known as the ex uno principle (or the “rule of one,” qāʿidat al-wāḥid) of Avicenna’s metaphysics, that “From the One, only one proceeds” (lā yaṣdur ʿan al-wāḥid illā l-wāḥid).1 Therefore, according to this philosophical dictum, God is the direct cause of only one effect and the indirect cause of everything else.2

Avicenna attaches great importance to this principle in his metaphysics. He uses it to prove the existence of the immaterial intellects (al-ʿuqūl al-mujarrada) and explain how the world proceeds from its principle. Indeed, of all the arguments he presents to prove the existence of the intelligible realm, only the proof based on the “rule of one” takes the form of a demonstration of the reasoned fact (propter quid, burhān limmī). My aim in this paper is principally twofold: first, to understand the logical underpinnings of the ex uno principle and how it is justified by Avicenna through a close examination of the supporting arguments he provides from a range of texts in his oeuvre; and second, to appreciate how widely discussed and debated Avicenna’s procession-argument is over its longue durée during the post-classical period of Islam (ca. 1100–1800).

To be sure, none of the arguments for the existence of the intelligible world stirred greater interest among Avicenna’s commentators than the ‘rule of one’ argument. In the 6th/12th century, the argument was subjected to severe criticism by al-Ghazālī (d. 505/1111), Abū l-Barakāt al-Baghdādī (d. ca. 560/1165), Sharaf al-Dīn al-Masʿūdī (fl. 582/1186) and Fakhr al-Dīn al-Rāzī (d. 606/1210). Al-Rāzī’s critique in particular became the prime conduit through which seventh/thirteenth-century figures such as Athīr al-Dīn al-Abharī (d. 663/1265), Naṣīr al-Dīn al-Ṭūsī (d. 672/1274) and Najm al-Dīn al-Kātibī (d. 675/1277) engaged Avicennan philosophy, albeit that from the eighth/fourteenth century onwards discussions surrounding Avicenna’s philosophy and the proofs he had formulated for the Intellects became increasingly—though not entirely—absorbed within the commentarial tradition(s) around major kalām manuals such as al-Mawāqif of ʿAḍud al-Dīn al-Ījī (d. 756/1355), the Sharḥ al-Maqāṣid by Saʿd al-Dīn al-Taftāzānī (d. 792/1390) and, most significantly, the Tajrīd al-iʿtiqād of Naṣīr al-Dīn al-Ṭūsī. It was not until the Safavid period that earlier attempts at confuting the ‘rule of one’ were resisted by Mīr Dāmād (d. 1041/1631) and his student Mullā Ṣadrā (d. 1045/1635), who each made the Neoplatonic scheme of emanation a firmly entrenched doctrine within their respective philosophical systems.3

1 Avicenna and the Rule of One

Avicenna treats the question of how the many relate to the One in all his major writings. The most important for our purposes to begin with is his discussion in Book IX, Chapter 4 of the Metaphysics (al-Ilāhiyyāt) of the Shifāʾ, for it is here that the problem is treated in terms of the very nature of the First Principle (al-mabdaʾ al-awwal). Avicenna begins by going through some preliminary observations that have already been discussed and established in earlier parts but which are needed in order to advance the discussion, particularly in respect of certain judgments concerning the First’s nature that have a direct bearing on Its acts and their metaphysical appraisal. Foremost among these is Avicenna’s appropriation of the Neoplatonic principle that simplicity and necessity are co-implied; all things that are unqualifiedly simple are necessary in themselves and vice-versa.4 Just as the First’s intrinsic necessity precludes It from having any internal causes, so too must its intrinsic necessity prevent It from having any external causes; for that which is necessary of existence in itself is by its very nature precluded from having any causes whatsoever. It is because of this that Avicenna then immediately explains why the acts of the First Principle cannot be based on prior intention (qaṣd) as this implies not only that the Necessary Existent acts for the sake of (and is therefore perfected by) something other than Itself, but also because “this would lead to a multiplicity in His essence.”5 It is due to the reciprocal nature of things being unqualifiedly simple and necessary in themselves that Avicenna denies not only there being any form of multiplicity in the First Principle, but also any form of multiplicity issuing forth directly from It either. Indeed, any procession of multiplicity from the One implies a multiplicity in the One. We shall see his arguments for this shortly.

Avicenna then proceeds to reject the opinion in which the First is denied the knowledge of Its own effect(s) such that whatever proceeds from It does so “by nature” (bi-l-ṭabʿ), and furthermore disregards the opinion that these effects are to Its own dissatisfaction.6 As an intellect the First apprehends Itself, and in so doing knows that It is the Principle of the effects that proceed from It.7 Furthermore, since no imperfection or impediment restrains Its activity the First is entirely satisfied with the effects generated through Its own activity; the world is good because its Principle is Good.8 Finally, Avicenna stresses that the entire procession of being from the First is a reflection of Its own intrinsic necessity, so that even though the generated effects are contingent by virtue of being inherently dependent on the First for their existence they are nevertheless necessary ab alio through their principle.9

Having explained here in brief and elsewhere in greater detail that the effects of a Necessary Being are themselves by way of necessity, Avicenna avers that “[i]t is not possible for the first of the existents proceeding from Him [i.e. by necessity] […] to be many, either in number or through divisibility into matter and form.”10 Appreciating what Avicenna says here requires a brief pause. His aim is clearly to deny two forms of multiplicity, what I shall call “extrinsic” and “intrinsic” multiplicity. The first type of multiplicity (“extrinsic”) is when two things, say, exist, and by virtue of their being two they are regarded as multiple or many. This is true even in cases where the things in question belong to the same type or species, as, for example, when two human beings are enumerated as being two in number. The second type of multiplicity (“intrinsic”) is properly speaking reserved for divisibility. An essence that is capable of being resolved into parts is divisible (munqasim), composite (murakkab) and multiple (mutakaththir bi-l-dhāt). Avicenna denies both forms of multiplicity vis-à-vis the first effect of the Necessary Existent so that neither (a) compositeness in nature (i.e. intrinsic multiplicity) nor (b) a plurality in number (i.e. extrinsic multiplicity) are conceivable for whatever proceeds from the First directly, or else the First would not be necessary of existence in itself according to Avicenna’s rationale. Put differently, since whatever emerges as a direct result of the First’s activity is a reflection of its principle, and because that principle is essentially necessary, the first effect must also mimic to the greatest extent possible the necessity and simplicity of the First Principle, and therefore must also be necessary of existence (albeit by another) and simple (albeit having an essence).

As the reflection of its principle, Avicenna therefore urges that had the first effect not been simple then the First would not have been necessary of existence in itself, and hence what the argument seems to hinge upon is the First’s self-sufficiency. That this is so is borne out by the fact that Avicenna expends great effort to demonstrate how, if the First were a direct cause of multiplicity, It would have to be composite (murakkab) and thus contingent. If all this is correct, then Avicenna’s “rule of one” seems justified: from the One, only one proceeds.

Bringing these points altogether, Avicenna concludes:

It has become evident, then, that the first of the existents [proceeding] from the First Cause is one in number, its essence and nature being one and not in matter. Hence, nothing either of bodies or of the forms that are the perfections of bodies is a proximate effect of Him. Rather, the first effect is a pure Intellect, because it is a form not in matter. It is the first of the Separated Intellects that we have enumerated, and it seems to be the principle that moves the outermost sphere by way of [being the object] of desire.11

Avicenna expressly states here that the first effect of the Necessary Existent is an Intellect, an immaterial substance whose role is to serve as a final cause for the eternal motion of the outer sphere. He has shown how, considering what is said here and in his earlier remarks, that the only effect to proceed from the First directly is an immaterial Intellect. As for the remainder of the section (al-Shifāʾ, al-Ilāhiyyāt IX.4), Avicenna is less concerned with the procession of the First Intellect than he is with the procession of multiplicity itself. In other words, given that simple causes make simple effects, Avicenna’s attention is now turned towards an explication of how multiplicity itself is generated from this first effect; for if the First Intellect is simple and itself subject to the “rule of one,” then this would seem to imply that there can be no multiplicity save in that respect whereby one intellect engenders another. The existence of bodies and souls thus requires an explanation, hence Avicenna’s lengthy treatment of the problem in the remainder of this section.

However, the emergence of multiplicity qua multiplicity does not concern us in this paper. What is relevant in these discussions is how Avicenna defends the “rule of one” with supporting arguments in different writings; three are provided in total. The first occurs in the Ilāhiyyāt of the Shifāʾ and a further two arguments in his Mubāḥathāt and Taʿlīqāt respectively, all of which are extensively debated in the post-Avicennan period. Before turning to his critics’ and commentators’ views, therefore, let us first present these arguments as they appear in the Avicennan works.

(1) Avicenna’s first supporting proof appears in the same section in which his discussion of the ex uno principle at Ilāhiyyāt IX.4 of the Shifāʾ appears. This also happens to be his only supporting argument in al-Ishārāt wa-l-tanbīhāt and al-Najāt, and given the importance of these writings one might reasonably say it is therefore Avicenna’s main argument. He begins by considering the possibility that multiplicity proceeds directly from the One, so that what proceeds from It is either (a) two things simple in nature but differing in subsistence (e.g. intellect and soul) or (b) a composite thing (e.g. a body composed of matter and form). The first scenario ([a]) exemplifies what Avicenna calls “multiplicity in number” (kathra bi-l-ʿadad), since neither the soul nor the intellect in this case are numerically identical. The second scenario ([b]) exemplifies Avicenna’s notion of “multiplicity in essence” (kathra bi-l-inqisām) given that a body in which matter and form are combined is intrinsically composed. Avicenna thus covers all possibilities by considering both intrinsic and extrinsic forms of multiplicity and aims to demonstrate how neither of their processions from the First directly is conceivable; for what ultimately ensues from them is a multiplicity within the First Principle. As this is impossible for the One, the original premise from which this conclusion arises must therefore be judged false and its contradictory true. The argument is thus set up as a proof by contradiction (qiyās al-khulf) designed to subvert the notion that any form of multiplicity whatsoever proceeds as a direct consequence of the First Principle.

To demonstrate this, Avicenna argues that both scenarios entail a multiplicity of “relations” or “aspects” (sing. jiha, ḥaythiyya) given that in the first scenario the First’s relation to the intellect cannot be the same as Its relation to the soul, nor in the second scenario can Its relation to matter be the same as Its relation to form. If this is so, then the question arises as to what it is that has caused this multiplicity; i.e., whence have these multiple aspects and relations derived from? Avicenna considers two possibilities: that each aspect has its existence grounded upon a factor that is either internal or external to the First. If that which gives rise to the multiplicity of relations is itself due to internal factors distinct from one another in divinis then this would necessitate the First being a composite, an impossible outcome for Avicenna. But if the factors are both external (“necessary concomitants,” lawāzim) then this merely begs the question of how these lawāzim themselves came into existence, and thus pushes the question of their issuance a step further in the chain of causes. So, the argument repeats itself until eventually one is forced to accept one of two absurd conclusions: that either the chain of causes and effects regresses ad infinitum and as a result there is no actual first effect whatsoever; or that the First is composite. Both options are false according to Avicenna, meaning that the premise which gave rise to them must also be false. Hence, by returning to the premise at the top of the argument and declaring it as false, Avicenna derives the true conclusion, “from the One, only one proceeds.”12

(2) The first argument takes for granted that the One is a direct cause of multiple effects through multiple aspects, only to show in the end that this is a false assumption. To buttress the argument further, Avicenna adapts it by supplying a modified version of the proof in the Taʿlīqāt and the Mubāḥathāt to demonstrate that it would be just as impossible, if not more so, for the First to be a cause of multiple effects through a single aspect, thereby eliminating all the potential ways of considering the procession of the many from the One. A close examination of Avicenna’s writings shows that he in fact supplies two distinct arguments for this purpose, which may therefore be considered separately. The first is based on the idea of necessitation (ījāb) and Avicenna’s claim that a thing’s existence is preceded by it first being made necessary.13 As he explains in the Taʿlīqāt,

It is not possible that from a thing that is one and simple there should proceed anything save that which is one; for you have come to realize that nothing can come into being from another until its existence from that thing has become necessary. If therefore the procession of one thing from another becomes necessary, and then from that [same] aspect from which the first thing became necessary something else proceeds other than the first thing, then the procession of that first thing would not have become necessary [contrary to our hypothesis, and hence there is a contradiction]. If however [the source from which things proceed] is not a simple reality, then it is possible that [two things] proceed from it. If one thing proceeds from the aspect of its nature and another thing from the aspect of its will, then the discussion would concern the duality of nature and will and their necessitation and procession from something simple. […] Hence it is not possible for there to be a multiplicity of any sort in that being which is Necessary of Existence.14

I assume that Avicenna’s purpose here is to show why two or more things cannot proceed from the One simultaneously through a single aspect, though I admit that the passage could be interpreted slightly differently. Nevertheless, it seems clear to me that Avicenna’s point is that the First Cause has only a single aspect because of Its simplicity, and that due to this fact alone only one being can be made necessary of existence. I suppose the reason for this is that multiple beings require multiple aspects for them each to become necessary, but since ex hypothesi there aren’t any other aspects to speak of there cannot be any other beings besides the one that has already been necessitated, which therefore proves Avicenna’s assertion that from the One only one other being proceeds directly.

If I am correct, then what this passage is primarily concerned with is demonstrating why a single aspect cannot bear the ontological burden of causing multiple effects. Avicenna’s claim is simply that if the One has only one aspect, then this either necessitates A’s existence and therefore makes B’s existence impossible, or vice versa. But let it be assumed that A has been made necessary of existence and that despite this B happens to be the one that proceeds from the First. Avicenna surmises that in such a situation there is a tension, because presumably a single aspect can only necessitate one existence. As such, A’s existence would have been made both necessary and impossible of existence—and that is a contradiction.15 The conclusion, as Avicenna goes on to remark, is that a multiplicity of effects requires a multiplicity of aspects, since each aspect relating the First to Its act can only be held to account for the necessitation of one effect only. The argument is thus reverted to the argument in the Shifāʾ which, as shown previously, results in the conclusion, “From the One, only one proceeds.”

However, it could be argued that there is a grave mistake in Avicenna’s reasoning here given that it seems to take for granted the very principle it sets out to prove; namely, that multiple effects require multiple aspects. Because Avicenna focuses on just a single aspect, however, he assumes that multiple effects cannot all be necessitated together, which is clearly question-begging. Indeed, it is unclear why based on this argument alone B’s necessitation makes A’s existence impossible. On its own, the argument does not appear to stand.

(3) Avicenna himself perhaps recognized the shortcomings of his previous argument and thus sought other ways of proving his point, possibly leading him to formulate his third supporting argument this time in the Mubāḥathāt. Like the one before it, this too is concerned with the generation of multiplicity through a single aspect, except this time Avicenna ignores the idea of effects first being made necessary of existence altogether and instead demonstrates much more precisely why multiple effects cannot proceed from a single aspect. To do this he considers two effects, X and Y, which are both hypothetically posited as two simultaneous effects of the First. Avicenna thus considers for the sake of argument, as averred by nearly all the mutakallimūn, that the many proceeds directly (bi-lā wāsiṭa) from the One. Consistent with his claim in the Ishārāt that differing realities must be conceptually different also, Avicenna argues that if X and Y are different both in concept (mafhūm) and reality (ḥaqīqa), then it would be accurate to say that ‘F causes X’ and ‘F causes Y’ from a single aspect. But since Y is an instance of the concept (mafhūm) ‘not-X,’ Avicenna thinks it also accurate for the second proposition to be reconfigured as ‘F causes not-X.’ What is interesting is that he goes on to claim that these are now incompatible propositions: ‘F causes X’ and ‘F causes not-X’ are, according to Avicenna, real contradictories. Therefore, since only one of these propositions is true, “from the One, only one proceeds;” either X or not-X, but not both.16

Unlike the argument in the Ilāhiyyāt (IX.4) of the Shifāʾ, Avicenna is no longer concerned about the implications, if any, the procession of the many would have on the One. Rather his purpose now is to show that the very idea of the many qua many somehow proceeding from a single aspect is itself inconceivable. Whatever the case, all three arguments entail impossible conclusions: the first entails that either the One is a composite (murakkab) or leads to the absurd result that no effect whatsoever proceeds from It; the second to the false conclusion that a single effect is both necessary and not necessary at the same time; and the third to the “contradiction” that F is simultaneously a cause of X and not-X. All three arguments take the form of a proof by contradiction and are thus aimed with the purpose of rejecting the initial hypothesis that the many proceed from the One. So, by converting this starting premise, Avicenna can assert his desired conclusion: “From the One, only one proceeds.” Avicenna uses this derived conclusion to subsequently argue that only an intellect can proceed from the First. As such, the “rule of one” itself then becomes the premise of yet a further argument to demonstrate by a process of elimination that the only effect to proceed directly from the Necessary Existent is an immaterial substance. Matter and form cannot exist separately from one another and must, therefore, come into existence simultaneously, which according to all these arguments already implies the presence of multiplicity. A body which is likewise composed of matter and form has multiple aspects within itself that disallow its procession from the First without intermediaries. As for soul, it cannot exist without body, and therefore cannot be regarded as the first effect of the Necessary Existent. Hence, the Intellect is the only being that is neither multiple in essence nor number, and therefore the only being that emanates directly from the Necessary Existent.17 Whatever proceeds subsequently, does so by the intermediation of the First Intellect.

2 The Rule of One in the Post-classical Period

There is clearly much riding on Avicenna’s “rule of one”: the pre-eternity of the intelligible world, the restriction of God’s power to just a single immediate effect, and other controversial topics such as the eternal motion of the heavens are but some of the outcomes this principle props up within the Avicennan system. Those wishing to chop down the various “ugly” aspects of Avicenna’s philosophy could not have found a better and more convenient starting point in their attacks than this principle of Avicenna’s thought. It is after all one of the outstanding areas of Avicenna’s metaphysics in which the Neoplatonic scheme of emanation is integrated and robustly defended, and hardly surprising therefore that it was ferociously critiqued and debated after Avicenna.

Before turning to the Islamic reception, a comment about the different styles of scholars’ engagement with the “rule of one” seems in order. When grappling with the criticisms of Avicenna’s critics it is important to make a distinction between (1) those which attack the principle and its surrounding proofs directly, and (2) those which focus their attack on the efficacy and potentially false implications of that principle further down the line in the emanationist scheme. It may be the case that Avicenna is inconsistent in his use and application of the “rule of one” or even that the principle is judged as being correct but nevertheless inadequate for the purposes which Avicenna intends for it.18 Yet whichever tactic is adopted, counter-arguments of this type generally speaking go beyond the principle itself by factoring in other considerations which may as a consequence undermine the original Avicennan claim. That being said, my focus in this paper is just on the principle itself and its supporting arguments. Indeed, if, as we shall see, an author criticizes Avicenna because he thinks Avicenna contradicts his own principle, then this is not so much an objection to the “rule of one” as it is an indictment of Avicenna’s consistency, or so it might be claimed.

2.1 Al-Ghazālī (d. 505/1111)

This indirect way of critiquing the “rule of one” is clearly evinced in Avicenna’s chief critic, the Ashʿarite theologian Abū Ḥāmid al-Ghazālī (d. 505/1111). The gist of al-Ghazālī’s objections to the ex uno principle in the Third Discussion of his Tahāfut al-falāsifa is that Avicenna cannot have it both ways: either he (a) commits to the “rule of one” steadfastly and denies the procession of any intrinsic forms of multiplicity so that the only things to ever proceed from the First are a series of eternal Intellects one after the other, each prior Intellect being the cause of the one after it19—an outcome Avicenna clearly rejects; or he accepts that either (b) some degree of multiplicity must be admitted in the First Itself,20 or—contrary to his principle—(c) that composites proceed from the First directly. For al-Ghazālī, only the second and third options guarantee the possibility of non-intellectual substances such as bodies and souls coming into existence. Furthermore, given his own Ashʿarite stance on God and His attributes, it is not so much the “rule of one” that is being questioned in al-Ghazālī’s critique but rather the First’s simplicity. Indeed, for al-Ghazālī the principle falters precisely because it is questionable whether the subject (mawḍūʿ) of the principle—namely the One as Avicenna conceives It—even exists.21

At this juncture it is worth reminding ourselves that although Avicenna’s general aim in the Ilāhiyyāt (IX.4) is to explain how the entire cosmos proceeds from the First Principle, he nonetheless tackles this problem in two very distinct stages. In the first stage, Avicenna’s goal is to demonstrate why it is impossible for bodies, souls and material forms to proceed as direct effects of the First Principle. He does this by arguing from the very nature of the First Principle and Its simplicity and necessity, showing that only another simple being that is “like” the First proceeds directly from the One. In the second stage, Avicenna recognizes that the “simplicity” of the first effect cannot be identical in every respect to the First’s “simplicity,” and hence some degree of equivocation is inevitable. This is a crucial step, else no real multiplicity emerges. Avicenna acknowledges therefore that for the soul and body of the outermost sphere to be generated, some degree of multiplicity perforce must be introduced at the level of the first effect in such a way that this is not itself the reflection of a prior multiplicity in the First Principle. Otherwise, he either confutes his own ex uno principle or inadvertently denies the First being a true One.

Avicenna’s solution to this delicate conundrum is to posit multiple acts of intellection (taʿaqqul) or analytical reflections (iʿtibārāt) in the first effect which, because the First does not think in a discursive fashion, cannot occur in the One. In this way, Avicenna introduces some element of multiplicity—albeit of an intellectual kind only—in the first effect (i.e. the Universal Intellect), which he emphatically states has no corresponding equivalent in the First Principle. It is through the productive quality of these intellectual acts that successive orders of multiplicity acquire their existence. Accordingly, the First Intellect performs three acts of intellection. By reflecting on its principle, the Necessary Existent, the First Intellect engenders the existence of the Second Intellect, and by reflexively thinking itself as both (a) necessary of existence by another and (b) possible of existence in itself, it bequeaths the existence of the first celestial soul and body, respectively.22

The point of saying this is to highlight that out of the five objections raised by al-Ghazālī to Avicenna’s emanationist cosmology, four are concerned with Avicenna’s second purpose in Ilāhiyyāt IX.4 and as such do not target the “rule of one” directly. Al-Ghazālī is thus mainly concerned with Avicenna’s account of how successive orders of multiplicity emerge after the first effect. By showing that the Avicennan account of how lower beings come into existence is defective, al-Ghazālī has enough ammunition to target the procession of existence from the First Principle Itself. The argument focusses on the nature of simplicity and composition and whether “multiple” intellections in the first effect constitute an instance of intrinsic multiplicity. If, as Avicenna holds, the emergence of the Second Intellect, the celestial soul and body of the outermost sphere require three acts of intellection in the First Intellect, then the simple fact that the products issuing from these acts are discrete existences is enough to convince al-Ghazālī that the corresponding acts of intellection performed by the first effect cannot all be one and the same in extenso. That is, the First Intellect’s thinking itself cannot be the same act of thinking as when it thinks the First Principle, for example. But if on the contrary all three acts of intellection amount to just a single act, then on what basis can Avicenna claim that three distinct existences emerge from one act of intellection by the First Intellect? Is this not itself a clear instance of Avicenna subverting his own “rule of one”?

What al-Ghazālī is ultimately gesturing here is that if the first effect’s three acts of intellection are all one and the same but nonetheless sufficient to cause the generation of the many, then why posit the existence of intermediary intellects in the first place? For surely God Himself in whom the subject (al-ʿāqil), act (al-ʿaql), and object (al-maʿqūl) of intellection are identical—as Avicenna believes—suffices for the procession of composite beings directly.23 Al-Ghazālī thus questions Avicenna’s consistency, for it seems the latter has one rule for God and another rule for the First Intellect.24

2.2 Sharaf al-Dīn al-Masʿūdī (fl. 582/1186)

Heavily influenced by al-Ghazālī and Abū l-Barakāt al-Baghdādī, the Bukhara scholar Sharaf al-Dīn al-Masʿūdī is the first post-Avicennan thinker to engage with the “rule of one” directly. Citing tanbīh V.11 from the Ishārāt, al-Masʿūdī explains that Avicenna’s aim is “to explain how things [come into] existence from the First Principle in the order that is necessary for them, and how despite their generic, specific and individual differences they all terminate in a single Principle.”25 But this aim, says al-Masʿūdī, cannot be fulfilled without two prior axioms of Avicenna’s metaphysics: (1) the principle that that which is One in every respect has only one thing proceed from it (i.e. the “rule of one”) and (2) that the Necessary Existent is one in every respect. As I have indicated above, al-Masʿūdī flatly rejects the second claim. As far as al-Ghazālī and al-Masʿūdī are concerned, the Avicennan One doesn’t exist.

Al-Masʿūdī grapples with the “rule of one” directly when he begins to focus on the first of his two mentioned axioms. He starts with a question directed not at the One from which things emanate, but the “one” that proceeds from the First Principle. “Is this ‘one,’ ” he writes, “one in species or one in number?” If it is one in number, then this implies it is simple and the only one of its kind, because there are no others like it. If it is one in species then it possesses a single nature, but this alone does not preclude others like it from existing also nor does it negate its being a divisible essence from another perspective.26 Avicenna clearly regards the first effect to be one in number; al-Masʿūdī rejects this description of the proceeding “one.”

He writes,

If you say that the proceeding “one” is one in number only, then this cannot be admitted and there is no indication for it in what you have mentioned. For if A and B are two individuals belonging to a single species, then it is conceivable that both are necessitated from a single aspect. A difference in aspect and relation would only be incumbent if the two proceeding things differed in essence and nature.27

This seems to miss Avicenna’s point entirely. Regardless of whether the effects belong to the same species or not, the fact remains that the notion of the First being a cause of A is different from the notion of It being a cause of B. So, evidently there are two aspects, not one.

If, on the other hand, the first effect is one in species, then al-Masʿūdī believes this claim can be confuted by the nature of being (wujūd) itself. For being qua being is “one” inasmuch as every existent qua existence is the same; the only difference among existents is that they possess different essences. Therefore, just as the light which emanates from the Sun is the same regardless of how many rays proceed from it and how many objects receive this light, likewise the wujūd that emanates from the First is the same irrespective of the inherent differences in the essences of the things receiving it. As al-Masʿūdī says, “the things [that receive existence] differ in terms of their natures (ḥaqāʾiq), not their existences, for they are caused things [only] inasmuch as their existences are caused and not their essences.”28 Apart from begging the question as to where these essences came from to receive existence in the first place, al-Masʿūdī nevertheless raises an interesting point about the subject of instauration (jaʿl) which, as we shall see, is discussed in a rather interesting way by Naṣīr al-Dīn al-Ṭūsī.

2.3 Fakhr al-Dīn al-Rāzī (d. 606/1210)

The Ashʿarite theologian Fakhr al-Dīn al-Rāzī discusses Avicenna’s “rule of one” in several of his writings. Of these, the al-Mabāḥith al-mashriqiyya is the most detailed and systematic. Al-Rāzī presents and critiques both sides of the debate in different sections of the book, and therefore provides a comprehensive treatment of the “rule of one” per se and the ensuing the question of how successive orders of multiplicity emerge after the first effect. Our focus in what follows is limited to the first discussion only. According to al-Rāzī, there have been four arguments that have been historically used as proofs for the metaphysical assertion that from the One only one proceeds. All four arguments make their way into later kalām discussions, and thus deserve to be considered one by one.

(I) The first is Avicenna’s “main” (Shifāʾ/Ishārāt) argument that whenever two things proceed simultaneously from the One, then either the One is not a simple reality or nothing at all proceeds from It given that an infinite series of simultaneous concomitant causes and effects (lawāzim) is impossible. What follows are six counterarguments by al-Rāzī against the conclusion of the first disjunct, namely, that every occurrence of multiplicity implies a composition in their original cause. Due to limited space, I shall consider just four of these.29

In his first rebuttal, al-Rāzī begins by asserting that the center of a circle is a terminus for a potentially infinite number of radial lines, each beginning at the circumference and ending at its centre. The centre is also a point; and according to the First Definition in Book 1 of Euclid’s Elements, “A point is that which has no parts.” However, considering Avicenna’s argument, whenever multiple things proceed (or, as in this case, converge) from a single point of origin (or, as in this case, point of convergence) then the point in question cannot be simple. In fact, since there are a potentially infinite number of radial lines there must be a potentially infinite number of corresponding parts in the centre, which is absurd. Hence, Avicenna’s argument is false.30

In the second rebuttal, he argues that if one were to take two simple realities A and B and consider them in the imagination jointly, then the notion of the set {A, B} that results from their conceptualization together would be different to our conceptualization of another set—e.g. {A, C}—in which a third simple reality is considered alongside A. Each set is conceptually different and distinct from the other, and by applying Avicenna’s own reasoning al-Rāzī concludes that all three simple realities would in fact have to be composite. But since the elements in question are all ex hypothesi simple in nature, Avicenna’s argument must be false.31

His third rebuttal also appears in his commentary on Avicenna’s Ishārāt and was the subject of an article published by Nicholas Heer more than twenty-five years ago.32 The objection here is that had the “rule of one” been valid, then the possibility of multiple negations from any simple being would also have to be denied. For example, our conceptualization that a certain thing is not a tree is different to our conceptualization that it is not a rock. Therefore, in the same way that the existence of multiple processions is either denied from the First simultaneously or, in the case that they do proceed from It, that the First is not a simple reality, likewise the case for multiple negations also: either they are impossible (in which case one cannot deny anything from the First) or the First is not a simple reality. Both options according to the falāsifa are false, and hence the “rule of one” must be false too.33

The fourth and final objection is al-Rāzī’s claim that the notion of a thing being-a-cause (al-muʾaththiriyya) is a characterization of it by way of relation (iḍāfa) to something else. The point al-Rāzī is making here is that it is only after we conceive two things being side by side that we then recognize from their mutual interactions that one thing precedes the other in existence, and that it, viz. the prior existent, is the cause of the existent that came after it, viz. the effect. But “cause” and “effect” are mental correlatives and are not, therefore, intrinsic characterizations and determinations of the very essences of the things themselves. Hence, being-a-cause (al-muʾaththiriyya) and being-an-effect (al-muʾaththariyya) are descriptions belonging to the category of relation (maqūlat al-iḍāfa). “And all [philosophers] concur,” says al-Rāzī, “that a multiplicity of relations does not lead to [the thing related] being multiple in essence.”34

Out of the six rebuttals, this is perhaps the most powerful. Its full significance is immediately obvious when related back to the wording of the “rule of one” itself. The verb “to proceed” (ṣudūr) that is mentioned in the statement “From the One, only one proceeds” is by al-Rāzī’s reckoning a relative term, which characterizes how two things, A and B, are related to one another. We say, for example, that “B proceeds from A,” thereby making A the thing-from-which-another-thing-proceeds (al-maṣdar) and B the-thing-that-proceeds (al-ṣādir). However, since being-a-maṣdar and being-a-ṣādir are relations, they needn’t imply a composition in the very essence of that thing from which other things proceed, or for that matter in the proceeding things themselves. Hence, it is perfectly reasonable to posit multiple relations (and thus multiple processions) from the First without this impinging on Its simplicity. Underpinning this argument is the idea that relations (al-iḍāfāt) are analytical posits (iʿtibāriyya ʿaqliyya), not actual existential determinations. In other words, the characterization of A “being-a-maṣdar” and of B “being-a-ṣādir” are not real properties of either A or B, but rather mental considerations.

(II) The second proof that al-Rāzī mentions and then criticizes is Avicenna’s argument from the Taʿlīqāt. Avicenna argued that if the First causes the procession of A and B from a single aspect, then It simultaneously causes the procession of A and not-A along a single pathway, which he claims is a contradiction.35 Avicenna’s wording is problematic. Logically speaking two propositions are contradictory when their qualities are different, i.e. one is affirmative and the other its negation. However, the propositions “F causes A” and “F causes not-A” are both affirmatives and thus conceivably false at the same time. Because of what appears to be a clear misunderstanding in Avicenna’s judgment, the passage provides a perfect opportunity for al-Rāzī to launch a scathing attack.

This kind of talk is so obviously wrong that even the weakest minds recognize it as being false. I have no idea, then, how something so obviously false has confused those who claim precise thinking. Indeed, it is baffling to me that someone [Avicenna] spends their entire life teaching the logical arts and studying them for the purpose of having a tool which prevents them from error, that when they are confronted with a matter of such noble importance they abandon its use and fall into error that even children mock at them.36

As we will see below, not all later thinkers agree with al-Rāzī’s stringent reading of the Avicennan text despite its prima facie inconsistency.

(III) The third of al-Rāzī’s four arguments is one of the most important, historically speaking. Although I cannot elaborate on it in the depth it deserves here, it nonetheless has far-reaching consequences in later discussions on the nature of the one and the many. It is given special importance by mystically inclined philosophers such as Mullā Ṣadrā who, being inspired by the unitary metaphysics of Ibn ʿArabī’s commentators, utilize it to argue in favour of a kind of ontological continuity and existential symmetry between God and creation, thereby making the existence of the First Principle somehow preserved in, and continuous with, the existences of Its effects, albeit in Ṣadrā’s case through a metaphysics of “modulation” (tashkīk).

Al-Rāzī expounds on this view by using the word “harmonious” (lāʾim) to describe the close resemblance between effects and their causes. As he writes, “The cause must be harmonious with the effect, since each of us conceives a degree of harmony between fire and heat which we do not find between, say, water and heat.”37 If at a general level it is true that every effect resembles its cause and is harmonious with it, the question arises what impact if any this has on the First Principle, which for argument’s sake causes two different realities from a single aspect. Accordingly, the First and Its two effects would all need to be harmonious. For al-Rāzī, this is nonsensical. How can the First be harmonious with Its effects and yet the effects themselves not be in harmony with each other? He begins the rebuttal by first clarifying that the notion that two things are harmonious just means that they are in some sense “similar” (mumāthil). In that case, the First is like Its effects in either a complete or partial way. If similar in a complete manner, then two absurd consequences follow: first, that neither the First nor what proceeds from It is more worthy of being regarded the cause of the other, but also—second—that two things which are in all respects similar and thus indiscernible are not two things at all but just one thing. On the other hand, if the First is like Its effect in some respects and not others, then this automatically suggests there are a multiplicity of aspects within the First, thus destroying Its simplicity. Hence, the First is neither similar nor harmonious with Its effect in any respect whatsoever.38

(IV) In his final proof for our purposes, al-Rāzī explains the rationale behind Avicenna’s “rule of one” by making an analogy with observable phenomena. He begins the argument by considering a body that is first heated by fire and then cooled by water. The resulting rise and fall in temperature are described as two effects occurring to the body, one due to its contact with fire and the other because of its contact with water. Since the fire’s effect on the body is different from the water’s, it follows that the natures of fire and water cannot be the same; or else they would not have affected the body differently. Therefore, the falāsifa conclude that because the multiplicity of effects is a sign that their causes are different, wherever there are multiple effects there must be multiple causes.

For al-Rāzī, this argument should also be rejected because of its failure to distinguish the notions of ikhtilāf and takhalluf. The former signifies a numerical difference among individuals of the same species, while the latter signifies a difference among things due to a difference in their species. The upshot of the argument is that multiple effects entail multiple causes only when the effects are different by way of takhalluf, not ikhtilāf.39 If Zayd laughs, ʿAmr cries, and Khālid learns to play the violin, then all these “effects” arise because of the fact they are all human beings, and thus a single nature suffices for multiple effects. Hence, there is no necessary entailment (mulāzama) between the effects being just numerically different and the inferred conclusion that their causes are multiple. More will be said about this argument below.

2.4 Naṣīr al-Dīn al-Ṭūsī (d. 672/1274)

In his commentary on Avicenna’s Ishārāt, the Shīʿī philosopher Naṣīr al-Dīn al-Ṭūsī defends the Avicennan “rule of one” against each of his major critics from the previous century, from al-Ghazālī at the beginning all the way through to Fakhr al-Dīn al-Rāzī at the end. In other writings and personal correspondences composed later in his career however, especially those which were produced after the Mongol capture of Bagdad in 1256 and the beginning of his association with the Ilkhanids, al-Ṭūsī appears to take a more reticent view. For example, in the Tajrīd al-iʿtiqād he describes each of the arguments advanced by the falāsifa to prove the existence of the intelligences (including Avicenna’s procession-argument) to be weak (madkhūla),40 though he does state elsewhere in the same work that there is equally no proof that they are impossible.41 I will start with the commentary on the Ishārāt and then consider some of his remarks in the Tajrīd. Needless to say, a full consideration of al-Ṭūsī’s opinions across all his writings is beyond the scope of this paper.

Al-Ṭūsī begins his comments on Avicenna’s tanbīh (V.11) by first clarifying that the meaning of the key term “one” appearing on either side of the exemptive particle illā in the “rule of one” in fact means different things. As a description of the First Principle, “One” in the first occurrence (i.e. before illā) signifies the notion of a thing being essentially one, having no composition or multiplicity. As a description of the first effect (i.e. after illā), however, “one” signifies the notion of a thing being numerically one, thus designating it as the only instance of its kind in existence.42 In other words, the entire species of whatever is called numerically one is contained in its single instance—as happens to be the case, according to Avicenna, with respect to all the intelligences. By offering this initial clarification about the conceptual (sing. taṣawwur) distinction between each of the “ones” on either side of the procession principle, al-Ṭūsī replies to the question initially raised by al-Masʿūdī in his Shukūk. This is an important step; without a correct taṣawwur of what the “rule of one” is saying, one cannot assent (taṣdīq) to it in the way intended by Avicenna.

Al-Ṭūsī then turns to the actual argument itself. He agrees with Avicenna’s reasoning and states that if the First were a cause of two things, then the aspect of It being the cause of one is not the same conceptually as It being the cause of the other, and hence “the difference between these two ideas points to a difference in their two realities, implying that what was presumed to be [essentially] one is not in fact [essentially] one.”43 Otherwise, says al-Ṭūsī, the First would either be two things in essence or something to which two different attributes are attached, in which case the being of the First is comprised of an essence and attributes. Both scenarios imply that the First is not a true One. By considering each of the permutations listed by Avicenna—namely that the effects proceeding from the First are either constituents (sing. muqawwim), concomitants (sing. lāzim), or a combination thereof—, al-Ṭūsī concludes by saying that “whenever two things result from something simultaneously, and neither is an intermediary for the other, then the thing from which they proceed is a divisible reality.”44 Avicenna’s entire processionist cosmology rests on this conclusion. If correct, one can apply the rules of immediate inference to infer the proposition known as the “rule of one.” The conclusion obviously cuts two ways, however: either (1) one affirms the True One and denies It being the cause of simultaneous multiplicity (Avicenna and the falāsifa); (2) or one affirms the direct procession of the many qua many and denies the True One (al-Ghazālī and the Ashʿarite mutakallimūn).45

So, having explained the key terms and premises of Avicenna’s tanbīh, al-Ṭūsī turns his attention to the objections raised by Avicenna’s critics by first giving rejoinders to two of al-Rāzī’s objections. The first is a rejoinder to his predecessor’s objection in Sharḥ al-Ishārāt in which he argued that if Avicenna’s “rule of one” were correct, then it would also have to hold in the case of negations (“From the One, only one is negated”) and receptions (“To the One, only one is predicated”). However, this is clearly false. As for negations, the First is neither a tree, nor is It a rock. And as for predication, “a man is capable of sitting or standing” and “a body is either black or in motion” are both truthful statements, thus indicating the possibility of their subjects receiving multiple predicates. Hence, the “rule of one” is false.

From al-Ṭūsī’s perspective, the “noble commentator’s” counter-examples betray a clear lack of understanding. Indeed, as al-Ṭūsī remarks, each of al-Rāzī’s objections is metaphysically flawed for the simple fact that they each take for granted an already present and existent multiplicity. But since this isn’t the problem which Avicenna is concerned about, it seems al-Rāzī has missed the point entirely. One cannot explain the origins of multiplicity by seeking principles which themselves are many, since this merely begs the question. So, regardless of whether we take the examples he provides in Sharḥ al-Ishārāt or the example of the centre-point of a circle being the terminus of multiple radial lines in the Mabāḥith, they are all equally metaphysically flawed. Each presumes that many things already exist, and thus appear to miss the point of Avicenna’s “rule of one” which seeks to explain the origins of this many in the first instance.

Al-Ṭūsī combines all al-Rāzī’s objections and duly dismisses them in one fell swoop in a single passage:

The response is that (a) negating something from something, (b) predicating46 something of something else, and (c) one thing being received by another—none of these is realized from the existence of one thing and one thing alone. None of them in fact results from something that is one qua one, but rather requires the existence of more than one thing preceding them, and hence from these [multiple] things [which are prior] these [other] things [which are posterior] follow through different considerations. Indeed, the procession of many things from many things is not impossible.47

In his second rejoinder, al-Ṭūsī tackles an objection from al-Rāzī’s Mabāḥith. As shown above, al-Rāzī raised an important objection arguing that the notion of ṣudūr and its two participles maṣdar and ṣādir are relations, and as such are extrinsic descriptions of the First Principle and Its effect, respectively, which do not impinge on their essences directly, thereby allowing the possibility of multiple things proceeding from God without causing multiplicity in the essences of either of the correlatives. Al-Ṭūsī responds by clarifying that the word ṣudūr can be used in two different senses, either (1) as a relative term predicated extrinsically of causes and their effects when occurring together, just as al-Rāzī intimated; or (2) as an intrinsic term denoting the very nature and being of the cause itself, so that the cause—either as a whole due to its essence or because of a state occurring to it—is characterized as something from which a specific effect proceeds.48 While admitting that the first meaning of ṣudūr is predicated posteriorly of each of the cause and effect’s existences, the second is a prior determination of the cause, characterizing it as a maṣdar irrespective of whether it is related to an effect or not. In other words, the second meaning of ṣudūr is predicated of the cause independently of the effect’s procession from it and before the effect is even existent. Al-Ṭūsī’s language is technical and convoluted, though crucial in terms of the Peripatetic system in which causes and effects are regarded as being essentially connected; i.e. the existence of the effect is already somehow contained in the nature of its cause and is a necessary consequence of it. Fire produces heat not simply because it has an accidental and haphazard relation to heat, but rather because the nature of fire is precisely such that given the right conditions it must necessarily produce heat. To deny this would in fact be a denial of secondary causation altogether, perhaps therein lying a clue as to the motivation of some of Avicenna’s Ashʿarite critics. As it appears in the “rule of one,” the term ṣudūr does not therefore refer to a relational property of the First and Its effect, but rather the very essence of the First Principle and Its first effect.

At this juncture it is worth shedding light on one of the most fiercely contested questions in the post-Avicennan tradition, already subtly alluded to by al-Masʿūdī in his Shukūk, concerning the precise causative relation a cause has to its effect in terms of the latter’s essence (māhiyya) and existence (wujūd). As is well known, at the core of Avicenna’s metaphysics is the idea that all contingent beings are composites of essence and existence. That being so, the question arises as to which of these is given to the effect by the cause in a primary and essential way in the process of “making” (jaʿl) the effect. Does the cause “make” the effect’s existence or its essence when it causes the effect? Logically speaking, there are only four options:

  1. the effect’s wujūd is the thing-made (majʿūl);

  2. the effect’s māhiyya is the thing-made;

  3. both are made; or

  4. neither is made.

The last of these possibilities is clearly impossible. If a cause neither makes its effect’s māhiyya nor its wujūd during the process of its instauration then it has not made the effect tout court. For many, the third option is just unfeasible given that it destroys the existential unity of extramental beings, not to mention the fact that it also leads to the problem of existence being superadded to quiddities in re and thus entails a well-known objection to the extramental supervening of existence. Therefore, the only viable options are (1) and (2). The importance of this debate can be immediately appreciated given the fact that the First Intellect is a contingent being and therefore a composite of essence and existence. This means that if both its essence and its existence were represented in re, then a simultaneous multiplicity would have proceeded from the First Principle, which although Avicenna states is impossible nonetheless presents a major challenge to the validity of the “rule of one.”

Al-Ṭūsī is acutely aware of this problem. What appears to have led him to consider it in the detail he has is perhaps instigated by his reading of al-Masʿūdī’s Shukūk, where there is a subtle hint that Avicenna’s procession principle may inadvertently backfire on Avicenna himself. If, as Avicenna believes, the First Intellect is a possible being having both quiddity and existence, and if, as Avicenna also believes, the mafhūm of a thing’s māhiyya is other than the mafhūm of its wujūd, then clearly one may ask whether this duality too emerges from the One Itself. Indeed, the very argument Avicenna provides to establish the “rule of one” in the Ilāhiyyāt of the Shifāʾ and in the Ishārāt could itself be applied in this instance to either undermine the simplicity of the First Principle or deny the procession of any first effect whatsoever. For Avicenna, both consequences would be catastrophic.

Al-Ṭūsī responds to this potentially devastating argument in a historically significant passage which although not appearing under the tanbīh in question, nonetheless addresses the problem at hand:

When something proceeds from the First Principle, that proceeding-thing has an ipseity (huwiyya) that is different from the ipseity of the First Principle.49 As a result, the notion (mafhūm) of “it” being something-proceeding from the First Principle is other than the notion of “it” being something possessing a certain identity. Hence, there are two intelligible meanings here: (a) the thing-emanating from the First, which is called existence (wujūd); and (b) the identity [of this proceeding-thing] which is a concomitant of that existence called quiddity (māhiyya). This quiddity, insofar as existence [is concerned], follows that [original] existence [which emanated from the First Principle]. The reason for this is that if the First Principle did not produce anything then there would not have been any quiddity whatsoever [option (4) above]. However, conceptually speaking (fī l-ʿaql) the existence [of the First Intellect] follows the quiddity [of the First Intellect in mental existence] given that existence is a [mental] property of quiddities.50

That al-Ṭūsī has the problem of jaʿl in mind is confirmed in Quṭb al-Dīn al-Rāzī’s super-commentary. He explains that in al-Ṭūsī’s view, indicative of Avicenna’s view also, the existence of the contingent effect is what is originally made (majʿūl bi-l-dhāt) by the cause in the process of instaurating the effect; and that its quiddity is a follow-on concomitant of that existence and therefore only accidentally made (majʿūl bi-l-ʿaraḍ).

The popular view (al-madhhab al-mansūb ilā l-qawm) is that quiddity is not made, but rather it is existence which is made. Therefore, it is existence which in reality proceeds from the First Principle. As for quiddity, its actualization in the external world is through [the actualization of] existence. Therefore, the quiddity [of the First Intellect] is an accidental act [mediated by the act of its existence], while the true act is the act of existence. If something proceeds from the [First] Principle, even though that thing shall have an ipseity, i.e. a quiddity, what really proceeds from It is existence. This is averred on the basis that quiddities are not made, and existence is different from quiddity […]. Existence and quiddity are thus two acts, except that one of them—namely, existence—is essentially made, while the other—namely, quiddity—is accidentally made.51

The “rule of one” also applies to the Avicennan essence-existence distinction, therefore. The two aspects of a contingent being proceed from their cause through different prior-posterior relations, and hence involve the procession of one thing through its intermediary. What proceeds directly from the cause is existence, and is thus—according to Mullā Ṣadrā’s nomenclature—the “fundamental” (aṣīl) ontic reality through which the quiddity of a thing is made. A lot more could and needs to be said here about the problem of jaʿl and its historical significance in the post-classical period, and about the precise meaning of the term iʿtibārī which is used to describe the nature of quiddity in later Islamic philosophy, especially in the thought of Mullā Ṣadrā. Suffice to say, our purpose here is merely to show that al-Ṭūsī had already anticipated this problem in terms of its relevance to the proof of the First Intellect. As far as the “rule of one” is concerned, only the existence of the First Intellect proceeds directly from the First Principle, and hence “From the One, only one proceeds.”

2.5 After al-Ṭūsī: A Brief Sketch

In his commentary on ʿAḍud al-Dīn al-Ījī’s (d. 756/1355) al-Mawāqif, the Timurid theologian al-Sayyid al-Sharīf al-Jurjānī (d. 816/1413) observes that in order to derive the conclusion “From the One, only one proceeds,” Avicenna must first demonstrate that the converse of this statement is true. In other words, if he can demonstrate that multiplicity is always the outcome of multiplied causes, then Avicenna can easily apply the rules of immediate inference to derive his desired conclusion.52 To explain, let us recall that in each of his “proofs” for the procession principle, Avicenna needed to consider what impact, if any, a simultaneous procession of multiple effects from the One would have on the very nature of this First Principle. Since the argument is based on a proof by contradiction, Avicenna’s aim throughout each of his supporting arguments is to show that whenever multiple effects proceed simultaneously from the First, either by way of multiple aspects or just one aspect, then this always undermines the original claim that the First is a simple being. Hence, there is a contradiction in either (a) the nature of the First Itself or (b) in the effects proceeding from It. Since both outcomes lead to impossible conclusions, Avicenna can confidently assert that the original premise (i.e. the many proceeds from the One) is false; and hence by conversion can declare his desired conclusion, “From the One, only one proceeds.”

Avicenna’s ‘rule of one’ is thus derived from the following initial proposition:


Whenever the effect is multiple, the cause is multiple (kullamā taʿaddada l-maʿlūl taʿaddadat al-ʿilla).

And by conversion:


Whenever the cause is not-multiple, the effect is not-multiple.

This can be re-worded as:


Whenever the cause is one, the effect is one (kullamā ittaḥada l-ʿilla ittaḥada l-maʿlūl).

If P is true, then C1 and C2 must be true also.

Al-Ṭūsī presented the same argument in his Tajrīd al-iʿtiqād.53 Given the significance of this work and its author’s reputation, the lemma of al-Ṭūsī’s argument stimulated myriad responses and debates in the commentaries and super-commentaries of the Tajrīd, thus making it a useful source for collating some of the different opinions on the procession principle by a host of different thinkers in the later medieval period. The first to criticize al-Ṭūsī was his student, the Shīʿī mutakallim al-ʿAllāma al-Ḥillī (d. 726/1325). For al-Ḥillī, the proposition that simple causes produce single effects is a particular judgment applicable to causes acting by way of necessity only (bi-l-ījāb) and not by free choice (bi-l-irāda), and thus cannot be generalized to cover all simple causes. While the procession principle may therefore in theory be applicable to the Necessary Existent in Avicenna’s theology, it is inapplicable to the God of traditional kalām, who unlike Avicenna’s First Principle is a volitional agent (fāʿil mukhtār). A freely choosing agent is thus excluded from the remit of the “rule of one” according to al-Ḥillī, who insists that the many proceed directly from the One and hence there is no reason to posit the existence of the First Intellect.54 Like Fakhr al-Dīn al-Rāzī, al-Ḥillī also comments that the notions of ṣudūr and taʾthīr are non-existential concepts, they are iʿtibāriyya, and therefore do not cause the real multiplicity Avicenna fears would result if the First was a direct principle of multiple effects.55 When, therefore, the notion of procession per se is an analytical consideration, a multiplicity of iʿtibārī processions would not undermine the First’s simplicity. Hence the original proposition (P) from which the procession principle is derived itself turns out to be false.

Another objection to the original lemma in al-Ṭūsī’s Tajrīd is identical to al-Rāzī’s objection in the Mabāḥith; namely, that the reason why a cause becomes multiplied in essence is not merely down to the fact that the effects proceeding from it are numerically multiple, but rather because they are essentially different. One cannot simply infer from the sheer numerical multiplicity of effects (ikhtilāf) that their cause is multiplied in essence (takhalluf).56 As such, the mulāzama in Avicenna’s hypothetical argument is only valid when the effects are different in their essences, but not in that case when effects, although multiple in number, belong to the same species. Multiplicity per se cannot therefore be the true logical indicant (dalīl) that the cause of each is itself multiple, and hence there is no truth—unqualifiedly speaking—to the claim that multiple effects are caused by multiple causes. Al-Ṭūsī’s fifteenth-century commentator ʿAlī Qūshjī, relying on al-Sayyid al-Sharīf al-Jurjānī, thus remarks as follows:

Supposing we were to see multiple effects without any [essential] difference [between them], still it would not be possible for us to infer the conclusion that the cause is multiple, for indeed that is the very thing which is being disputed.57

To deflect objections of this kind against Avicenna’s Ishārāt and Shifāʾ arguments, one may in the end have to revert to Avicenna’s modified argument in the Taʿlīqāt. Although two effects in the same species are essentially the same, each would still be an instance (miṣdāq) of the other’s negation. Hence, the First would still have to be and not be the cause of a thing’s existence from a single aspect, which is still a contradiction. It is quite plausible therefore that Avicenna himself formulated the Taʿlīqāt argument as a response to exactly this kind of objection.

However, as we have seen, the argument is controversial and problematic. It is rejected by al-Rāzī—and before him Bahmanyār—on the basis that the propositions “F is a cause of A” and “F is a cause of not-A” are not exactly contradictory statements. In his commentary on al-Rāzī’s Mulakhkhaṣ, Najm al-Dīn al-Kātibī (d. 675/1277) attacks Avicenna further by arguing that even if Avicenna were to be given the benefit of doubt, still his conclusion could not be granted. What if, al-Kātibī argues, Avicenna’s real intention was just the same as al-Rāzī’s own suggested corrective and that the contradictory of “F causes A” is “F does not cause A”? Is there now a contradiction? Al-Kātibī responds in the negative because neither proposition, he states, has a specified temporal quantifier, and so it is conceivable that both propositions are true albeit at different times.58

In the fifteenth century, a response to these objections was given by the Timurid philosopher-theologian Jalāl al-Dīn al-Dawānī (d. 908/1502) in his glosses on al-Qūshjī’s commentary on al-Ṭūsī’s Tajrīd. Al-Dawānī observes that if the First Principle causes the procession of a not-A, and hence the proposition “F causes a not-A” is true, then it would still be correct to say, “F did not cause A” because A is not not-A. More exactly, al-Dawānī writes that the proposition “F causes not-A” is itself an instance of the proposition “F did not cause A” and thus, properly speaking, a contradictory of “F causes A.”59 On this interpretation, al-Dawānī claims that Avicenna’s reasoning is perfectly accurate. As for al-Kātibī’s objection, this too according to al-Dawānī has no real bearing on the issue. Two apparently contradictory and unqualified propositions can still both be true if the times in which they are true are different, but false when the time in question is the same. However, as al-Dawānī points out, a discrepancy in times requires two different aspects, which therefore begs the question where this multiplicity has originated if the First Principle ex hypothesi has just a single aspect.60 Furthermore, the context of the discussion here is with reference to an atemporal procession given that the Intellects, like their Principle, transcend time. Al-Dawānī is alluding to the fact that God and the First Intellect are pre-eternal, not in the sense that each is an everlasting being but rather that they transcend time altogether. Had Avicenna temporalized his propositions then he might have given the misleading impression that God and the First Intellect are in time and therefore temporal.

Avicenna’s procession argument is a powerful and enduring one, being recognized as such even by his detractors throughout the post-classical era. It calls attention to the fact that within the Neoplatonic contours of Avicenna’s philosophy the metaphysics of causation operates not only along an asymmetrical path, effects being always inferior to their causes, but also maintains a degree of proportionality between causes and their effects. From these initial premises, Avicenna fashions a metaphysically robust theory of how the many proceeds from the One. To do so, he employs a rich repertoire of metaphysical concepts and manoeuvres which are original to his own philosophy. Indeed, of all the historical arguments presented for the existence of the celestial Intellects in his oeuvre, the procession principle is the only one to proceed from first principles. Inspired by his Neoplatonic predecessors, Avicenna formalizes the concept of emanation through a rigorous form of systematic reasoning. Whereas Plotinus and other Arabic falāsifa had to rely on metaphor to express how things come into being from the One, Avicenna supplants linguistic imagery with robust logic, and thus revives the insights of the sages of old with renewed vigour. However, as I have tried to show in this paper, not everything was plain sailing. Avicenna’s arguments are open to criticism and appear to falter in important ways, which as a result stimulated intense debate and criticism. The conclusions he draws from the “rule of one” are clearly repugnant to the classical kalām tradition. Yet, despite the scathing criticisms of his early commentators, there was a burgeoning school of thought outside of kalām that embraced the Avicennan “rule of one,” all the while adapting it to fit in line with their mystically inclined worldview. Taking their inspiration from the Andalusian mystic Muḥyī l-Dīn Ibn ʿArabī (d. 638/1240), members of this school adapted the procession principle in accordance with their doctrine of the unity of existence (waḥdat al-wujūd), thereby envisioning new and interesting ways in which this principle could be used to support their version of a truly unitary metaphysics whereby the only thing to proceed from God, the Sublime Essence in Its Singularity (al-dhāt al-aḥadiyya), is “God” Himself. The entire cosmos is thus “God in manifestation” since, according to this mystical interpretation of Avicenna’s principle, “From ‘God’, only ‘God’ proceeds.” This, however, is a story for another occasion.


An earlier version of this paper was presented at the Post-Classical Islamic Philosophy workshop at the Finnish Institute in Rome in December 2017. I would like to express my deepest thanks to Professor Jari Kaukua and the anonymous reviewer for their valuable criticisms and suggestions.


This is a Neoplatonic thesis, though the wording of the principle is itself Avicenna’s. For modern studies on this principle, see Cristina D’Ancona, “Ex uno non fit nisi unum. Storia e preistoria della dottrina avicenniana della Prima Intelligenza,” in Per una storia del concetto di mente, ed. by E. Canone (Firenze: Olschki, 2007), 29–55; Nicholas Heer, “Al-Rāzī and al-Ṭūsī on Ibn Sīnā’s Theory of Emanation,” in Neoplatonism and Islamic Thought, ed. by Parviz Morewedge (Albany, NY: SUNY Press, 1992), 111–26; Arthur Hyman, “From What Is One and Simple Only What Is One and Simple Can Come to Be,” in Neoplatonism and Jewish Thought, ed. by Lenn E. Goodman (Albany, NY: SUNY Press, 1992), 111–36.


Arabic philosophical and theological works usually express this in terms of a cause (here God) acting “with intermediaries” (bi-wāsiṭa) or “without intermediaries” (bi-lā wāsiṭa). A key concern among later mutakallimūn is whether the notion of God’s enacting things by way of an intermediary is a restriction of His power (qudra). For most, if not all, mutakallimūn, God is powerful and therefore able to enact all things directly. To suggest that His power is thus restricted to just one thing directly is seen as being tantamount to a rejection of God’s omnipotence.


Due to space limitations, I will only consider some of the debates up until the end of the tenth/sixteenth century.


Avicenna, The Metaphysics of the Healing (hereafter al-Shifāʾ, al-Ilāhiyyāt), ed. and trans. Michael Marmura (Provo, Utah: Brigham Young University Press, 2005) IX.4.1. (All citations will be to Marmura’s translation unless otherwise noted. I cite the book number in Roman numerals, followed by the chapter and paragraph numbers introduced by Marmura).


Avicenna, al-Shifāʾ, al-Ilāhiyyāt, IX.4.2.


Avicenna, al-Shifāʾ, al-Ilāhiyyāt IX.4.3; cf. idem., al-Mabdaʾ wa-l-maʿād, I.1, pp. 75–6.


Note however that according to Avicenna the First thinks in a non-discursive fashion, and in doing so is “free from duality” (al-Shifā, al-Ilāhiyyāt, IX.4.4). For a detailed analysis, see Peter Adamson, “Non-Discursive Thought in Avicenna’s Commentary on the Theology of Aristotle,” in Interpreting Avicenna: Science and Philosophy in Medieval Islam. Proceedings of the Second Conference of the Avicenna Study Group, ed. Jon McGinnis (Leiden and Boston: Brill, 2004), 87–111.


Avicenna, al-Shifāʾ, al-Ilāhiyyāt IX.4.3.


Avicenna, al-Shifāʾ, al-Ilāhiyyāt IX.4.4.




Avicenna, al-Shifāʾ, al-Ilāhiyyāt, IX.4.6 (Marmura modified); idem., al-Mabdaʾ wa-l-maʿād, I.3, p. 78.


Avicenna, al-Shifāʾ, al-Ilāhiyyāt IX.4.5, p. 328; cf. idem, al-Najāt min al-gharq fī baḥr al-ḍalālāt, ed. by Muḥammad Taqī Dānishpāzhūh (Tehran: Intishārāt-i Dānishghāh-i Tihrān, 1387Sh), 651–2; idem., al-Ishārāt wa-l-tanbīhāt, ed. by Maḥmūd Shihābī Khurāsānī (Tehran: Intishārāt-i Dānishghāh-i Tihrān, 1390Sh), namaṭ V, faṣl 11, p. 116 [= ed. J. Forget (Leiden: Brill, 1892), vol. 1, pp. 153–4; = ed. M. al-Zāriʿī (Qom: Bustān-i kitāb, 1434 AH), 287]; idem., Kitāb al-Hidāya, ed. M. ʿAbduh (Cairo: Maktabat al-Qāhira al-ḥadītha, 1974), III.3, pp. 274–5.


Avicenna, al-Najāt, 548–9.


Avicenna, al-Taʿlīqāt, ed. by Sayyid Hossein Mousavian (Tehran: Muʾassasah-i Pazhūhashī ḥikmat va falsafah-i Īrān, 1391Sh), 549 (nr. 966) [= Avicenna, al-Taʿlīqāt, ed. ʿAbd al-Raḥmān Badawī (Cairo: al-Hayʾat al-miṣriyya al-ʿāmma li-l-kitāb, 1973), 182].


Cf. Bahmanyār, Kitāb al-taḥṣīl, ed. M. Muṭahharī (Tehran: Dānishghāh-i Tihrān, 1375Sh/1996), 531.8–13.


Avicenna, al-Mubāḥathāt, ed. M. Bīdārfar (Qom: Intishārāt-i Bīdār, 1413 AH), 226 (nr. 673). Avicenna’s reasoning is clearly problematic. “F causes X” and “F causes not-X” are not contradictory propositions. This objection was raised possibly by Bahmanyār in his exchanges with Avicenna. Bahmanyār correctly observes that the contradictory of “F causes X” is “F does not cause X,” thereby indicating that the proposition “F causes not-X” is not incompatible with the proposition “F causes X.” Avicenna responds by arguing that our conceptualization of F being the cause of X is different to our conceptualization of F being the cause Y. Consequently, our differing conceptualizations, writes Avicenna, means that we ought to posit two different ontological aspects, one through which F causes X and another through which F causes Y. As such, neither aspect can be the same as the other, and thus multiple aspects are required for F to have multiple effects. The argument therefore reverts to Avicenna’s “main” argument.


Avicenna, al-Shifāʾ, al-Ilāhiyyāt IX.4.7–9; idem., al-Najāt, 652–4.


Abū l-Barakāt al-Baghdādī (d. ca. 560/1165) noted this point explicitly: “We say that this statement [i.e. ‘From the One, only one proceeds’] is true in itself, but neither does what they conclude from it follow nor can what they establish through it be based on it.” See Abū l-Barakāt al-Baghdādī, Kitāb al-muʿtabar fī l-ḥikma al-ilāhiyya, ed. Ş. Yaltkaya (Hyderabad: Dāʾirat al-maʿārif al-ʿuthmāniyya, 1358 AH), 3: 156.


Al-Ghazālī, The Incoherence of the Philosophers [Tahāfut al-falāsifa], ed. and tr. by Michael E. Marmura (Provo, Utah: Brigham Young University Press, 2000), III.39 (henceforth Tahāfut). I cite the Discussion number followed by the paragraph numbers introduced by Marmura. The exact same argument is made by Abū l-Barakāt al-Baghdādī, Kitāb al-muʿtabar, 3: 150–1.


See for example al-Ghazālī, Tahāfut, III.54.


This is particularly so when al-Ghazālī’s criticisms of the falāsifa’s proofs for God’s existence and unity are taken into consideration. Sharaf al-Dīn al-Masʿūdī (fl. 582/1186) intimates just as much in his critique-commentary (al-Shukūk wa-l-mabāḥith ʿalā l-Ishārāt) on Avicenna’s Ishārāt, remarking that the only thing proven through demonstration is that there is an uncaused First Cause. As for the nature of this cause being the necessity of existence and not having an essence (māhiyya) related to an existence, or the assertion that it possesses no entitative attributes that are pre-eternal and uncaused, then none of this can be proven by demonstration. See the Arabic edition of al-Masʿūdī’s commentary in Ayman Shihadeh, Doubts on Avicenna: A Study and Edition of Sharaf al-Dīn al-Masʿūdī’s Commentary on the Ishārāt (Leiden: Brill, 2016), 275–6.


Avicenna, al-Shifāʾ, al-Ilāhiyyāt, IX.4.11; cf. D’Ancona, “Ex uno non fit nisi unum.”


Al-Ghazālī, Tahāfut, III.53–9, 62.


Cf. Abū l-Barakāt al-Baghdādī, Kitāb al-muʿtabar, 3: 158–9.


Al-Masʿūdī, Shukūk, 275.


Cf. Avicenna, al-Shifāʾ, al-Ilāhiyyāt, III.2.4.


Al-Masʿūdī, Shukūk, 276.




For the entire section, see Fakhr al-Dīn al-Rāzī, al-Mabāḥith al-mashriqiyya fī ʿilm al-ilāhiyyāt wa-l-ṭabīʿiyyāt (henceforth Mabāḥith), ed. Muḥammad al-Baghdādi, 2 vols (Beirut: Dār al-kitāb al-ʿarabī, 1990), vol. 1, pp. 588–94; cf. idem., al-Maṭālib al-ʿāliyya min al-ʿilm al-ilāhī (henceforth Maṭālib), ed. Aḥmad al-Saqā, 9 vols. (Beirut: Dār al-kutub al-ʿarabī, 1987), 4: 381–9.


Al-Rāzī, Mabāḥith, 1: 590.3–19.


Ibid., 1: 590.20–3.


See Nicholas Heer, “Al-Rāzī and al-Ṭūsī on Ibn Sīnā’s Theory of Emanation.”


Al-Rāzī, Mabāḥith, 1: 590.24–591.18.


Ibid., 1: 592.1–3. Al-Rāzī is here highlighting the difference between primary and secondary intelligibles. His point is that the notions of “cause” and “effect” qua conceptualizations are secondary intelligibles, though he himself does not state this explicitly.


Fakhr al-Dīn al-Rāzī, Mabāḥith, 1: 589.1–2.


Ibid., 1: 593.1–593.5.


Ibid., 1: 589.3–589.5.


Ibid., 1: 593.6–593.20.


Ibid., 1: 593.25–594.4.


Naṣīr al-Dīn al-Ṭūsī, Tajrīd al-iʿtiqād, in al-ʿAllāma al-Ḥillī, Kashf al-murād fī sharḥ Tajrīd al-iʿtiqād, ed. Ḥasan Ḥasanzādah al-Amulī (Qom: Muʾassasat al-nashr al-islāmī, 1437), 278.


Ibid., 267.


Naṣīr al-Dīn al-Ṭūsī, Ḥall mushkilāt al-Ishārāt (henceforth Ḥall), in Avicenna, al-Ishārāt wa-l-tanbīhāt, 3 vols. (Qom: Nashr al-balāgha, 1435), 3: 122.15.


Ibid., 3:122.19–123.1.


Ibid., 3:123.2–3.


Abū l-Barakāt al-Baghdādī’s remark that even though “the rule of one” is prima facie correct it does not automatically imply a Neoplatonic cosmology is therefore accurate.


Reading ittiṣāf instead of ittiṣāl on line 6. This reading is confirmed by what al-Ṭūsī says in line 12.


Al-Ṭūsī, Ḥall, 3: 126.6–10.


Ibid., 127.2–6.


That is, even though both the First and Its effect can be referred to as an “it” (huwa), they are not the same; they are, so to speak, two “its.”


Al-Ṭūsī, Ḥall, 3: 245.10–5.


Quṭb al-Dīn al-Rāzī, al-Muḥākamāt, in: al-Ṭūsī, Ḥall, 3: 245 (fn. 1).


Al-Jurjānī, Sharḥ al-Mawāqif, ed. Maḥmūd ʿUmar al-Dimyaṭī (Beirut: Dār al-kutub al-ʿilmiyya, 2012), 4: 133.16–8.


The lemma of al-Ṭūsī’s text can be found in al-ʿAllāma al-Ḥillī, Kashf al-murād, 172.


Al-ʿAllāma al-Ḥillī, Kashf al-murād, 172; cf. idem., Kashf al-fawāʾid fī sharḥ Qawāʿid al-ʿaqāʾid, ed. Ḥasan Makkī al-ʿĀmilī (Beirut: Dār al-ṣafwa, 1413/1993), 261–4; idem., al-Asrār al-khafiyya fī l-ʿulūm al-ʿaqliyya (Qom: Būstān-i kitāb, 1387Sh/[2008]), 507–9, 541–3. Al-Sayyid al-Sharīf al-Jurjānī (Sharḥ al-Mawāqif, 4: 125) and ʿAlī Qūshjī (Sharḥ Tajrīd al-ʿaqāʿid, ed. by Muḥammad Ḥusayn al-Zāriʿī al-Riḍāʾī [Qom: Intishārāt-i rāʾid, 1393Sh], 1: 570) both support this view.


Al-Ḥillī, Kashf al-murād, 174–5. He also makes the rather interesting observation in passim that as analytical constructs (umūr iʿtibāriyya), the various aspects and relations Avicenna posits for the processions of successive orders of multiplicity cannot be the causes of real entities; for thought by itself cannot engender real existence.


Saʿd al-Dīn al-Taftāzānī, Sharḥ al-Maqāṣid, ed. by ʿAbd al-Raḥmān ʿUmayra (Beirut: ʿĀlam al-kutub, 1998), 2: 98.5–12; ʿAlī Qūshjī, Sharḥ Tajrīd al-ʿaqāʿid, 1: 575.3–576.4; al-Jurjānī, Sharḥ al-Mawāqif, 4: 137.1–7.


ʿAlī Qūshjī, Sharḥ Tajrīd al-ʿaqāʿid, 1: 576.3–4.


Najm al-Dīn al-Kātibī, al-Munaṣṣaṣ fī sharḥ al-Mulakhkhaṣ (MS Tehran, Kitābkhānah-i Millī 1640), fol. 277ª; cf. al-Jurjānī, Sharḥ al-Mawāqif, 4: 137.7–139.5.


For al-Dawānī’s rejoinder to al-Rāzī, see al-Qūshjī, Sharḥ Tajrīd al-ʿaqāʿid, 1: 576 (fn. 3).


Al-Dawānī in ʿAlī Qūshjī, Sharḥ Tajrīd al-ʿaqāʿid, 1: 576 (fn. 3).


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