Th emistius ( c . ad 317-88) taught at Nicomedia before he started a philosophical school in Constantinople, the city where he also served in the Senate. He entertained good relations with a series of emperors from Constantius II and Julian to Th eodosius I, to whom many of his orations are dedicated. 1 Among philosophers he is best known for his paraphrases of a number of Aristotle's works, which probably date to his early teaching career, and were written as part of his personal training -although they also seem to bear traces of use in his school. 2 His treatment of Posterior Analytics , Physics and On the Soul survive in Greek; in Hebrew, Latin and Arabic versions we have his On the Heavens and Metaphysics Lambda . 3 Modern scholars agree that his paraphrase of On the Soul is the boldest of them all. Th is paraphrase develops a distinctive theory of intellect that shows traces not only of Aristotle, but also of Plato's Timaeus , Th eophrastus, Boethus, * I wish to ac knowledge gratefully how much I have benefi ted from the critical comments of an audience in Oxford in 2015, followed up by extensive written comments from Richard Sorabji, and further audience at CEU in Budapest in 2017, who were the fi rst to hear about the four principles to be discussed below. I thank Anna Marmodoro and István Bodnár for their generous invitations. Needless to say, all errors that remain are mine. 1 See Kupreeva, 'Th emistius' for a recent survey of Th emistius' life and works. Th e current consensus is that Th emistius never wrote full commentaries in the style of e.g. Alexander of Aphrodisias, for which see Steel, 'Des commentaires'; Blumenthal, 'Th emistius: Th e last Peripatetic?'. Following custom I shall refer to Th emistius' paraphrases with In DA ( On Aristotle on the Soul ), In Phys. ( On Aristotle's Physics ) so as to distinguish them from Aristotle's works. 2 See Orations 23, 89.20-90.5, ed. Schenkl and Downey. Todd (ed.), Th emistius on Aristotle On the Soul 2 n. 13 draws attention to scholē at In DA 32,23 and 108,36 possibly meaning lecture; 39,23 mentioning pupils; and 40,4-5, 46,27-8 invoking previous instruction. Th emistius, On Aristotle's Posterior Analytics , ed. Wallies, 1.2-13 suggests a possible use by students who have read Aristotle once, and wish to return to his works without having time to study the more elaborate commentaries available at the time. 3 For editions and translations see the Bibliography. Brague (ed.), Th émistius: Paraphrase de la Métaphysique d'Aristote , conveniently combines the Hebrew and Arabic versions of the paraphrase of Metaphysics 12 in a single French translation. For an English version of relevant passages from that paraphrase see also Pines, 'Some Distinctive Metaphysical Conceptions'.