Physis A primary substance or element from which everything is thought to be derived.
Plato First a disciple of Socra- tes, came under the influence of the Pythagoreans, and postulated the existence of an abstract world of forms or ideas that, when manifested in matter, make up the objects in the empirical world. The only true knowledge is that of the forms, a knowledge that can be gained only by reflecting on the innate contents of the soul. Sensory experience interferes with the attainment of knowledge and should be avoided.
Protagoras A Sophist who taught that “man is the measure of all things.” In other words, what is considered true varies with a person’s personal experiences; therefore, there is no objective truth, only individual versions of what is true.