Greek Philosophy

The idealism of Socrates was organized by Plato into a systematic philosophy. In his theory of Ideas, Plato regarded the objects of the real world as being merely shadows of eternal Forms or Ideas. Only these changeless, eternal Forms can be the object of true knowledge; the perception of their shadows, that is, the real world as heard, seen, and felt, is merely opinion. The goal of the philosopher, he said, is to know the eternal Forms and to instruct others in that knowledge.

Plato's theory of knowledge is implicit in his theory of Ideas. He argued that both the material objects perceived and the individual perceiving them are constantly changing; but, since knowledge must be concerned only with unchangeable and universal objects, knowledge and perception are fundamentally different.

In place of Plato's doctrine of Ideas with a separate and eternal existence of their own, Aristotle proposed a group of universals that represent the common properties of any group of real objects. The universals, unlike Plato's Ideas, have no existence outside of the objects they represent. Closer to Plato's thought was Aristotle's definition of form as a distinguishing property of objects, but with an independent existence apart from the objects in which it is found. Describing the material universe, Aristotle stated it consists of the four elements, fire, air, earth, and water, plus a fifth element that exists everywhere and is the sole constituent of the heavenly bodies “above” the moon.

In the writings of Plato and Aristotle the dominant strains of idealism and materialism in Greek philosophy reached, respectively, their highest expression, producing a body of thought that continues to influence philosophical inquiry. Subsequent Greek philosophy, reflecting a historical period of civil unrest and individual insecurity, was less concerned with the nature of the world than with the problems in the individual. During this period four major schools of largely materialistic, individualistic philosophy arose: that of the Cynics, and those espousing Epicureanism, Skepticism, and Stoicism. For a detailed history of these and earlier schools, see Philosophy.