Abstract: Xenocrates, who had accompanied Plato on one of his visits to Sicily, became head of the Academy in 339 B.C. Xenocrates stays close to what he takes to be the cosmological doctrine of Plato's Timaeus; indeed Xenocrates’ doctrine may be seen as something of a retreat from Speusippus’ radical position, perhaps in response to Aristotle's criticisms. Dillon reconstructs Xenocrates's cosmological or metaphysical scheme as comprising a pair of first principles, the Monad, or Nous, and the Dyad, or the ‘Everflowing’, to which the Pythagorean tetraktys corresponds as the active counterpart; and a World-Soul, which receives the forms from the Supreme God's mind, and projects them upon the physical plane. In Logic, Xenocrates remained faithful to Platonic logic, rejecting the Aristotelian categories, although he did argue that the species was prior to the genus; in Ethics, while keen to formalize Plato's teachings, Xenocrates ends up with a position very similar to Aristotle's, in that he emphasizes the needs of the body as well as those of the soul. Xenocrates had a dominant effect on the development of Platonism, because he systematized what he took to be Plato's philosophical system, thus laying the foundation for the ‘Platonic’ system of philosophy; it is Xenocrates’ definition of Form, for instance, which became the standard definition of a Platonic Form.

Keywords: Aristotle, Dyad, genus, Monad, Nous, Platonic Form, Pythagorean, species, Timaeustetraktys, World-Soul,