The theory, or doctrine, of forms (also called the theory of ideas) is Greek philosopher Plato’s (c. 428–347 B.C.) expression of his belief that there are forms that exist outside the material realm, and therefore are unchanging—they do not come into existence, change, or pass out of existence. It is these ideas that, according to Plato, are the objects or essence of knowledge. Further, he posited that the body, the seat of appetite and passion, which communes with the physical world (rather than the world of ideas or forms), is inferior to the intellect. He believed the physical aspect of human beings to be irrational while the intellect, or reason, was deemed to be rational.

The origins of Plato’s theory can be traced to Socrates (c. 470–399 B.C.), who believed that the psyche (inner spirit) has intuitive access to divinely known principles or truths, which he attempted to formulate through his conversations with others. Indeed, the Socratic dialogues, written by Plato, reveal that Socrates was striving to define the exact nature of the traditional Greek moral virtues of piety, temperance, and courage.