Manuel Mertens
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This book discusses the magical and mnemonic writings of Giordano Bruno, a 16th-century philosopher who was burnt at the stake. When I have presented my research to laymen summarized thusly, it has often been welcomed with great enthusiasm. Not only do magic and mnemonics excite general interest, but his auto-da-fé on the Campo dei Fiori in Rome seems to make the whole enterprise even more alluring.

In March 2016 a colleague and I guided a group of seventeen-year-old students through Rome. On the first day, after visiting the Roman Forum and the Palatine hill, we reached the Campo dei Fiori at sunset. The day had been long and exhausting. Yet, despite their obvious fatigue, the faces of these boys and girls lit up when we paused at Bruno’s statue and started to recount his life story. Gazing into history, they could see how, on 17 February 1600, this cosy little square had been the setting for a cruel episode. This man they were looking at had opted to die rather than to renounce his own ideas. How crazy was that? And then there were his ideas, which were even crazier: an infinite universe with an infinite number of worlds, the transmigration of souls, a reform of magic; all these ideas well stored in fabulous mnemonic palaces. Who could think of anything more spectacular? Whereas my voice might have betrayed my admiration for this thinker, it was Bruno’s life story and his extraordinary way of thinking that triggered their fascination.

It goes without saying that listing the heretical points for which he was condemned only provides a partial answer to the question as to why this thinker was sentenced to death. Nowadays, one could fill many bookshelves with Bruno studies, and many of these studies propose an interpretation of his execution as a milestone in the history of thought, some more convincingly than others. This book, in turn, sheds new light on this episode. It leads to a quite literal reading of the words spoken by the philosopher himself on the day of his execution, showing that they were well balanced and rightly chosen. After having stated that those who sentenced him to death were probably more fearful than he who had to undergo the penalty, he proclaimed that “he died willingly, as a martyr, and that his soul would go up with the smoke to paradise”.1 Whereas his statement that “his soul would go up with the smoke to paradise” is generally read as an expression of his heroism, I will argue that, in magical and mnemonic terms, for Bruno a heroic state of mind could be described exactly as a fiery spirit elevating the soul to a divine level. In other words, his last words were uttered less metaphorically than they are usually read. However, to understand how exactly Bruno believed this to be true, it is necessary to get a grip on his magical and mnemonic ideas. And since a discussion of these principal topics that does not include their Neoplatonic background makes no sense, I shall briefly explain here what magic and mnemonics meant to Bruno, and how they relate to philosophy. Thereafter I shall present my overall argument with a short description of my four chapters.

A magical revival took place at the end of the 15th century, in great part thanks to Marsilio Ficino’s works, and his translations of Neoplatonic and Hermetic sources. Thus, at the moment the witch-hunts began to intensify, philosophical treatises emerged to offer a well-defined intellectual framework for the magical practices then coming into prominence. The Neoplatonic view of an animated world, in which similitudes linked the different levels of being, explained why plants and stones could contain occult virtues. These virtues were “occult” because their causes were hidden from human understanding. Nevertheless, their marvellous effects could be experienced and were considered to be dependent on the natural powers of heaven. By means of this universal sympathy, physicians could cure their patients by invoking the celestial powers with herbs, animals, and images associated with a given planet. The similarities between specific herbs, animals, and images and their ruling planets were thought to explain their capacity, as a kind of “bait”, to attract the desired planetary power and thus influence the lower world. Hence, the all-encompassing similitudes led to a conception of “causality” far removed from our present explanation of “why things happen”. Within this world view, a special role was reserved for the universal spirit, a thin, airy substance believed to mediate between the celestial and sublunary worlds and to facilitate the traffic of higher powers with the world below. By virtue of this spirit the magician was said to unite or “marry” heaven and earth. One could rightly argue that this universal spirit was inherited from the pneumatic doctrine of the Stoics rather than from the Neoplatonic school. A strict distinction between these philosophical currents, however, was not yet in force, certainly not before Justus Lipsius’s works on Stoicism. In brief, it was predominantly sources translated by Ficino which offered a way of philosophically underpinning a range of magical practices already present in the Middle Ages and coming closer to the surface in the Renaissance.

The importance of the Neoplatonic heritage does not mean that magical issues did not fit into early modern scholasticism. A great challenge, for example, was to reconcile Neoplatonic magic with the Aristotelian-Thomistic conception of substantial forms. If a magical preparation of matter (the incision of an image on a stone, or the pronunciation of words) did not alter the substantial form of this matter, how could it result in the manifestation of a property which was previously absent? For opponents of magic, this manifestation pointed to the intervention of a supernatural essence or demon. Hence, early modern scholasticism also broached magical issues. Charles Schmitt even spoke of an “invasion of Hermetic material into Aristotelian texts”, referring to the work of such Aristotelians as Agostino Nifo or, in England, John Case.2 Although magic cannot be identified with Hermetism, both subjects obtained an important position within the philosophical debates of the Renaissance. Many philosophers took a stance against superstition and witchcraft but did not consider all magic to be contemptible. On the contrary, sometimes a great value was ascribed to it. Giovanni Pico della Mirandola even described it as the fulfilment of natural philosophy (magia est consummatio philosophiae naturalis). By Bruno’s time magia naturalis was generally understood as the “practical part of natural philosophy” and was opposed to superstitious and theurgical magic, which, by invoking angels, came into conflict with orthodox religion. This last form of magic acquired a bad reputation because there was always the suspicion that the invoked angels or (in a Neoplatonic sense) benign planetary spirits were in fact evil demons, very adroit at disguising themselves and deceiving mere mortals.

These debates were still in progress when Bruno, a philosopher pur sang, concluded in the 1580s that an infinite cause like God should result in an infinite effect too: the infinite universe. Magic occupies a considerable place in Bruno’s oeuvre. But an initial remark is necessary. By propounding an infinite universe with an infinite number of worlds, where the centre was everywhere and the circumference nowhere (according to the famous formula from the Liber XXIV philosophorum), Bruno dissolved the traditional cosmological hierarchy, on which a great part of magic was based. Was his conception of magic, then, influenced by his new cosmological vision? In a word, how could he “marry” heaven and earth in an infinite universe?

Although many passages in his magical writings still echo the hierarchical conception of the world, there are also indications of an endeavour to adapt magic to his infinite universe. The central point is that, although the physical universe has lost its traditional hierarchy, the structuring of the mind – precisely one of the aims of his art of memory – remains indispensable for magical action. And that being the case, the traditional view of magia naturalis as the “practical part of natural philosophy” is not fully applicable to Bruno’s magic, in which cognition and operation converge. There is another reason why Bruno’s “natural magic” does not conform to the traditional notion of the term, as defined in opposition to demonic magic. Notwithstanding Bruno’s stress on the “natural” character of good magic, his conception of nature as matter imbued with spirit implies the existence of demons. These are corporeal beings consisting of very subtle matter or “spirits”, fully integrated into nature, where their actions are seen as “physical causes”. Hence, magical action, although natural, must take these causes into account as well.

This does not mean that, in line with the ongoing debates, Bruno does not reject superstitious forms of magic. From his first writings we encounter contemptuous references to such abracadabra. In his comedy Candelaio, for example, the old roué Bonifacio tries to seduce the courtesan Vittoria by means of magic. Another figure in the same comedy, Bartolomeo, devotes himself to alchemy in the hope of becoming rich. These characters and their superstitious forms of “magic” are ridiculed. One year later, a good form of magic is presented in Bruno’s Sigillus sigillorum as one of the four guides (together with love, art, and mathesis) of the inner (i.e. psychological) acts. Opposed to superstition, this kind of magic is called “a companion and rival of nature”, but also “in some way director and governor of nature for one’s own use”.3

However, it is a difficult exercise of the historical imagination to approach the concept of “superstitious” magic, or, even more so, its alleged rejection, without being unduly influenced by our own understanding of “superstition”. As an example we could refer to a passage of De magia naturali, in which Bruno tells of a sort of “nose transplant” to prove that the soul diffuses outside its body:

Experience teaches this also in the case of those whose nose has been cut off; if they arrange to grow a new nose for themselves from the flesh of some other animal, and if that animal whose flesh was used dies, then as the body of that animal rots, so does the borrowed nose. From this, it is clear that the soul diffuses outside of the body in every aspect of its nature.4

Odd testimonies of this sort are not alien to Bruno’s magical writings. We may wonder where and how he gained the “experience” of such a phenomenon, which we would immediately relegate to the domain of “superstition”. Although his overall reasoning shows a progressive view on magic, some elements which are in our view “superstitious” clearly remain.

A perceptive remark by Keith Thomas, whose studies have contributed much to the comprehension of the phenomenon of magic, may help us to understand these “superstitious remains” in Bruno’s magic. “It is a feature of many systems of thought”, Thomas writes in the concluding chapter of Religion and the Decline of Magic, “and not only of primitive ones, that they possess a self-confirming character. […] Such systems of belief possess a resilience which makes them virtually immune to external argument.”5 Many wizards or astrologers were able to explain the failure of their operations from within their own system of beliefs. The failure, they said, was due to a mistake in calculations, or the magus had omitted a vital ritual precaution. Maybe his power was simply not great enough, and therefore he could advise his patient to visit another, more powerful magus. This self-confirming attribute of certain systems of thought is well illustrated by a passage from Chaucer’s The Canon’s Yeoman’s Tale (402–408), where the failure of an alchemical experiment does not injure the belief in alchemical theory.

Another seyde the fir was over-hoot,
But, be it hoot or coold, I dar seye this,
That we concluden everemoore amys.
We faille of that which that we wolden have,
And in oure madnesse everemoore we rave.
And whan we been togidres everichoon,
Every man semeth a Salomon.6

Likewise, Bruno, often praised as a thinker well ahead of his time, continued to adhere to certain “systems of belief” which he had inherited, and in which his thought was formed from his youth onwards. His rejection of “superstitious” forms of magic should therefore be qualified according to his own principles. In our view, these principles relate to his art of memory. Superstitious magic implies a passive and credulous attitude on the part of those who are involved in the magical practice. Non-superstitious magic, on the other hand, is based on the regulated belief (regulata fides) of those who consciously act and are not acted upon against their will. Two of the major aims of his art of memory, as we will see, were precisely to regulate belief and to prevent a passive and credulous attitude.

Thus, after these introductory remarks on Bruno’s magic, let us look briefly at the art of memory. We find references to the ars memoriae in the chief rhetorical writings of antiquity – Cicero’s De oratore, Quintilian’s Institutio oratoria, and the pseudo-Ciceronian Rhetorica ad Herennium.7 From these treatises we understand that this art makes use of places (loci) and associative images (imagines) to remind the orator of his arguments. In a preparatory phase, the speaker places these images in the desired order within a well-known building. When he delivers the oration, he begins by taking an imaginative walk through the building, encountering the images that remind him of the arguments. In this way, he can reliably both remember his arguments and recall them in the proper order during his discourse.

It is only in the second half of the 20th century that the different forms taken by this ars memoriae in the various stages of Western history have been the subject of serious research, undertaken by scholars such as Paolo Rossi, Frances Amelia Yates, Mary Carruthers, and Lina Bolzoni. By now, the study of ars memoriae (from antiquity through the Middle Ages, and well into the early modern period) has become a fully recognized branch of the humanities – one which may even be said to be “in fashion”.

In the Renaissance the ars memoriae naturally persisted in a rhetorical context and was employed by those who had to compose orations or sermons. When Bruno entered the convent of San Domenico Maggiore in Naples in 1565, the Dominicans were known for their expertise in this art, which was considered useful in proselytizing. Trained in the art of memory, the missionary was armed with a large quantity of arguments with which to convert the infidel. From his own testimony we know that Bruno was acquainted with the art of memory at an early age, and was probably soon recognized for his mnemonic feats. In the early 1580s he gained access to the French court by attracting the interest of Henri III with his art of memory. The philosopher later pursued a career as a renowned mnemonic teacher. It is no overstatement to say that Bruno owed a great part of his career to his fame as a mnemonist.

Besides its use in a rhetorical context, the scope of the ars increased considerably in the 16th century. Giulio Camillo, for one, claimed that his art contained great secrets which were veiled in his theatre. A later mnemonist, Lambert Schenkel, promised almost superhuman results, including the ability to learn languages in a short span of time, and the capacity to dictate fifteen letters on different subjects at the same time. Bruno, for his part, clearly inserted the art into his philosophical project.

This project is explicit in De umbris idearum (1582), Bruno’s first surviving mnemonic treatise. Although many different philosophical currents are tapped when it suits the author, a primary position is ascribed to Plotinus, the Platonicorum princeps. Plotinus’ view of memory as an active power, through which the soul realizes its true, and divine, self, stands in clear opposition to the Aristotelian conception of memory as a receptive cognitive faculty. Plotinus’ view of memory makes Bruno’s introduction of mnemonics in a philosophical context understandable. The core of the philosophical project delineated in De umbris idearum is precisely to seek out divine ideas through their shadows (that is, their reflections in the human mind) – a neverending and never fully achievable quest. As such, Bruno’s art of memory seems to propose an epistemology in which memory plays a leading role.

But, as with “superstition”, here again we must be careful not to project our own category of “epistemology” onto Bruno’s. It would be wrong to see the art of memory as merely being concerned with epistemology. For as we have already seen, in Bruno’s mind cognition and operation are closely intertwined. In the Ars memoriae his art is described as following nature’s perfection and emulating its industry, but also as perfecting its shortcomings.8 This definition of his art of memory echoes the definition of magic given in his Sigillus sigillorum. The analogous definition of the arts of memory and magic offers an ideal starting point for our study, which proposes to clarify the relationship between Bruno’s writings on the art of memory and his magical works via a comprehensive study of their form and content.

In the book’s first chapter we will look at the major studies that have been dedicated to our subject. I will show that modern scholarship has been dominated by two currents, represented by Frances Amelia Yates and Rita Sturlese, the former arguing for a magical reading of Bruno’s art of memory, the latter denying that Bruno’s mnemonics are linked to magic. Besides these contributions, we also take into account more recent studies of this issue. Both currents are capable of adducing passages in Bruno’s mnemonic writings to confirm their own reading. This means that Bruno’s treatises contain apparently contradictory passages, that is, passages that affirm a magical reading and others that deny it.

In the book’s second chapter, therefore, I try to clarify these “contradictory passages”. We might be tempted by the approach offered by Leo Strauss’s thesis that persecution leads to a certain “art of writing” in which contradictions can play a part in the heterodox author’s strategy to avoid suspicion. Yet it goes without saying that from a methodological perspective, putting a microscope over the “art of writing” is a tricky enterprise. If the author’s opinion – located somewhere between the lines – is not clearly expressed, the way is open to an infinite number of interpretations. On the one hand, Bruno sometimes declares that he fears censure and that he deliberately writes in such a way as to not be understood by everyone. Since magic was certainly a dangerous bottle to uncork, it is not surprising that the link between his art of memory and magic is left implicit. On the other hand, his allusions to magic are sometimes too obvious and of an offensive nature. Why is there this ambiguity with regard to magic in his mnemonic works? This methodological difficulty requires a formal approach including both textual corpora. In the end, the ambiguity will become more understandable from a rhetorical point of view, rather than having it explained away as fear of persecution.

After this study of the form, the book’s two final chapters will shift their focus to the content of both corpora. The third chapter attempts to study the philosophical basis of both Bruno’s art of memory and his magic: the concept of similitudo. Firstly, I will seek a definition; then, I will show how both arts function by means of this concept; and finally, I will demonstrate how similitudo is used to express the aim of both the magical and the mnemonic doctrine.

In the book’s fourth chapter another key concept of both magic and mnemonics is taken into consideration: spiritus. After an introduction to this concept, the role of spiritus in Bruno’s psychology is analysed, especially in relation to his magic, which is concerned with the capacity of demonic spirits to deceive the cognitive faculties. We will then look back at Bruno’s early mnemonic treatises, where this concern for demonic deception was already present.

At the end the reader will have a view on a specific area of Bruno’s mind, where epistemological and magical issues are inseparably interconnected. Thus, an exceptional part of the history of philosophy will come to the surface in the works of an exceptional thinker.


L. Firpo, Le Procès, BOeuC, 1:523: “Giovedì mattina in Campo di Fiore fu abbrugiato vivo quello scelerato frate domenichino da Nola, di che si scrisse con le passate: heretico ostinatissimo, et havendo di suo capriccio formati diversi dogmi contro nostra fede, et in particolare contro la Santissima Vergine et Santi, volse ostinatamente morir in quelli lo scelerato; et diceva che moriva martire et volentieri, et che se ne sarebbe la sua anima ascesa con quel fumo in paradiso. Ma hora egli se ne avede se diceva la verità.”


C.B. Schmitt, Aristotle and the Renaissance (Cambridge, Mass.: Harvard University Press, 1983), pp. 97, 99–101.


G. Bruno, Sigillus sigillorum, BOMNE, 2:266: “[…] naturae cunctipotentis aemula et socia efficitur [magia] atque quodammodo eiusdem ad proprium usum directrix et gubernatrix.”


G. Bruno, Cause, Principle and Unity: And Essays on Magic, ed. and trans. R.J. Blackwell (Cambridge: Cambridge University Press, 1998), p. 113. Cf. G. Bruno, De magia naturali, BOM, p. 188: “Ipsum et experientia docet in ipsis qui, abscisso naso, novum sibi ex aliena carne succrescere fecerunt membrum; siquidem obeunte diem illo, cuius erat caro, iuxta modum quo putrescit corpus illius, etiam mutuatus nasus ille putrescit. Hinc manifestum est animam plus se diffundere extra corpus, per totum horizontem suae naturae.”


K. Thomas, Religion and the Decline of Magic: Studies in Popular Beliefs in Sixteenth- and Seventeenth-Century England (1971; repr. Middlesex: Penguin Books, 1982), p. 767.


The Works of Geoffrey Chaucer, ed. F.N. Robinson (London: Oxford University Press, 1974), p. 217.


Cicero, De oratore (Cambridge, Mass.: Harvard University Press, 1913), 2:350–67; Quintilian, Institutio oratoria (Cambridge, Mass.: Harvard University Press, 2002), 10:2; Pseudo-Cicero, Rhetorica ad Herennium (Cambridge, Mass.: Harvard University Press), 3:16–24.


G. Bruno, Ars memoriae, BOMNE, 1:122: “Tunc artem sub umbra idearum degere arbitramur, cum aut torpentem naturam antecedendo sollicitat, aut deviam exorbitantem dirigit et perducit, aut deficientem lassamque roborat atque fulcit, aut errantem corrigit, aut perfectam sequitur et industriam emulatur.”

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