Greek philosophy may be understood as a sort of running conversation among men attempting to rationally explain the world as a whole. To begin with Plato is to enter into the middle of a conversation that has already been taking place for quite some time, and in order to understand the importance of Aristotle we must briefly digress into the philosophies of two other men: Parmenides and Heraclitus. Stated succinctly, Parmenides (515-450 B.C.) and Heraclitus (535-475 B.C.) represent two competing and opposing views of the world that cannot be reconciled.
Parmenides’ argues that change, or motion, to use the traditional language, is impossible. Being can only be changed by something other than itself. But what is there besides Being? Non-being. Non-being, though, is not anything at all. Therefore, it cannot affect change of any kind. Being is, non-being is not; you will never get beyond this thought. What we then have is an image of the world where there is no multiplicity or diversity of things, there is just Being. This Being is absolute and eternally itself. Because of this, all of our apparent experiences of the world as subject to change are merely that, apparent. The world of change is an illusion.
Contrast this with Heraclitus’ philosophy of an ever-changing reality. For Heraclitus, everything is constantly in a state of change and nothing is ever actually static. All things are in a state of “becoming,” nothing can be said to “be” in a definitive sense. Accordingly, the world of permanence, like the world of change for Parmenides, is utter illusion. What is merely apparent to us is the world of things that have continual existence; the reality is that all is in flux.
Plato took these two adversarial ways of thinking very seriously, and, for him, the Forms were an attempt to answer the question of how the world can be both changing and unchanging, for both are given to common sense experience. Plato, however, set for himself yet another serious problem, one that he even addressed in the dialogue Parmenides. Forms exist, material things exist, but the interaction between the two is seemingly elusive. The attempt to account for a world of both change and permanence, then, still appears to be unsuccessful. That is, until Aristotle.