Both aspects of the genius of Socrates were first united in Plato of Athens (428-348 BCE), who also combined with them many the principles established by earlier philosophers, and developed the whole of this material into the unity of a comprehensive system. The groundwork of Plato's scheme, though nowhere expressly stated by him, is the threefold division of philosophy into dialectic, ethics, and physics; its central point is the theory of forms. This theory is a combination of the Eleatic doctrine of the One with Heraclitus's theory of a perpetual flux and with the Socratic method of concepts. The multitude of objects of sense, being involved in perpetual change, are thereby deprived of all genuine existence. The only true being in them is founded upon the forms, the eternal, unchangeable (independent of all that is accidental, and therefore perfect) types, of which the particular objects of sense are imperfect copies. The quantity of the forms is defined by the number of universal concepts which can be derived from the particular objects of sense.
The highest form is that of the Good, which is the ultimate basis of the rest, and the first cause of being and knowledge. Apprehensions derived from the impression of sense can never give us the knowledge of true being — i.e. of the forms. It can only be obtained by the soul's activity within itself, apart from the troubles and disturbances of sense; that is to say, by the exercise of reason. Dialectic, as the instrument in this process, leading us to knowledge of the ideas, and finally of the highest idea of the Good, is the first of sciences (scientia scientiarum). In physics, Plato adhered (though not without original modifications) to the views of the Pythagoreans, making Nature a harmonic unity in multiplicity. His ethics are founded throughout on the Socratic; with him, too, virtue is knowledge, the cognition of the supreme form of the Good. And since in this cognition the three parts of the soul — cognitive, spirited, and appetitive — all have their share, we get the three virtues: Wisdom, Courage, and Temperance or Continence. The bond which unites the other virtues is the virtue of Justice, by which each several part of the soul is confined to the performance of its proper function.
The school founded by Plato, called the Academy (from the name of the grove of the Attic hero Academus where he used to deliver his lectures) continued for long after. In regard to the main tendencies of its members, it was divided into the three periods of the Old, Middle, and New Academy. The chief personages in the first of these were Speusippus (son of Plato's sister), who succeeded him as the head of the school (till 339 BCE), and Xenocrates of Chalcedon (till 314 BCE). Both of them sought to fuse Pythagorean speculations on number with Plato's theory of ideas. The two other Academies were still further removed from the specific doctrines of Plato, and advocated skepticism.