The Plato κɛπαλαιον in Aëtius' chapter On Voice is the result of the interpretation, modernization, and systematization of brief passages dealing with hearing, voice and speech to be found in several dialogues. This construction of Plato's doctrine of 'voice' was mainly inspired by the systematic and innovative Stoic τóπος On Voice. The 'physical' definition is based on passages in Theaetetus and other works, the 'physiological' on a passage in Timaeus. The distinction and relation between voiceless internal λóγος (or thought) and spoken λóγος in Theaetetus and Sophist was interpreted as being equivalent to that between internal and uttered ϕωνη-cum-λóγος which played an important part in the Stoic view of the relation between thinking and speaking. Because as a rule Plato uses ϕωνη of the human voice, the rigorous distinction between this voice and that of animals and lifeless things postulated by Diogenes of Seleucia and other Stoics could be attributed to him, and his unsystematic usage justified by claiming that he used ϕωνη both in the proper and in a loose (or improper) sense. Approaches such as these are characteristic of Middle Platonism. In the present case the neutralization of Theophrastus' criticism of Plato in the De sensibus played a significant part. Plato's statement that thought is mirrored in what is spoken was updated by replacing it with a (fanciful) etymology of ϕωνη which must be dated to at least the Hellenistic period (it was known to e.g. Philo of Alexandria and used by the grammarian Philoxenus). Surprisingly full parallels for virtually the entire contents of the Aëtian κεϕαλαıον are found in the Commentaria in Dionysium Thracem. The etymology of ϕωνη, and others like it, were quoted and used by grammarians and lexicographers from the later first century BCE up to late Byzantine times. The attempt to understand the doxographer's lemma on Plato on voice thus becomes a case-study demonstrating both the openness and the tenacity of philosophical interpretation in antiquity. But note that the present inquiry is not concerned with the Aristotelian or (partly) Aristotelianizing tradition according to which language is conventional.One of the side-effects of the present inquiry was the unsurprising realization (again) that 'parallel passages', once quoted and interpreted out of context, may sort of drift from one book or paper to the next, while their interpretation hardens into received truth. In the present case the so-called parallels in Plato for the later distinction between the internal and the spoken voice proved to be not so parallel after all.
The role of Theodoret as a source for Aëtius has been questioned, or even denied, by Lebedev, Frede, Gourinat and Bottler. No one of course doubts that there must be a source P/S shared by ps.Plutarch and Stobaeus. But one can also prove in two steps that there must be a source T/S shared by Theodoret and Stobaeus. There are in fact 20 lemmata which Theodoret shares with Stobaeus, but are absent in ps.Plutarch, out of a total of about 100 lemmata which he records, i.e. about 20 % of the whole. As a next step one can prove that the P/S source and the T/S source cannot be distinguished from each other so must be identical. This has not been done before. Diels’ critics are refuted, and his original intuition that all three witnesses should be used to reconstruct a text of the Placita in a single column is vindicated. The fact that no one but Theodoret mentions Aëtius is due to the popularity of ps.Plutarch’s epitome. It is perhaps a pity that the work was not attached to a famous name as in the case of the other work and the epitome by ps.Galen. It might then even have survived unscathed. Giving the author another name is rather futile and there are good grounds for adhering to the scholarly tradition initiated by Diels. A brief appendix discusses recent papers by Lebedev and Schubert.