One of Plato's first pupils, Xenokratis, who was born in Chalcedon, followed Spevsippos as head of Plato's Academy, and held this position until his death. During his life he travelled with Plato and Spevsippos to Syracuse, learning the teachings of the Pythagoreans of Magna Graecia, and also travelled several times with Aristotle to Asia Minor for other philosophical studies.
He was known for his honesty and asceticism, and ate no meat. He was respected as a man of such perfect honesty that he was allowed to testify in the coursts without the usual sworn oath.
His many philosophical works deal mainly with ethics. Despite the list of some 75 titles of his works, found in the writings of Diogenes Laertius, only fragments remain. Along with ethics, Xenokratis was preoccupied with the nature of the gods and their relations with heavenly bodies, and combined Platonic and Pythagorean teachings.
He seems to have been the first to define three main branches in philosophical thought: physics, logic, and ethics, and similarly listed as three the levels of knowledge, as thought, sensory perception and opinion, which in turn, were associated with three forms of being: perceived, understood, and composite. He extended some of Plato's ideas, especially his theory of ideas, connected with numbers, conceiving of reality as the result of the interplay of the One (which represented unity, rest, and goodness), and the 'dyad', or Two (which represented multiplicity, motion, and evil). He gets a street named after him too in central Athens!