(died 314 BC, Athens) Greek philosopher. A pupil of Plato, he succeeded Speusippus (d. 339/338 BC) as head of Plato's Academy. His writings are lost except for fragments, but his doctrines, as reported by Aristotle, appear to resemble those of Plato. He divided all of reality into three realms: the sensibles, or objects of sensation; the intelligibles, or objects of true knowledge, such as Plato's forms (seeform); and the bodies of the heavens, which mediate between the sensibles and the intelligibles and are therefore objects of opinion. A second threefold division separated gods, men, and demons. He maintained that the origin of philosophy lies in mankind's desire to be happy, happiness being defined as the acquisition of the perfection that is peculiar and proper to mankind (seeeudaemonism).