Early in philosophy, there were two philosophers who did not agree on a certain subject. One was Parmenides and the other was Heraclitus. Parmenides claimed that change was impossible. Heraclitus claimed that everything changed nothing at all was unchanging. Heraclitus believed what he believed because he reasoned from what he experienced in the world. Parmenides had an argument. Once we know why that argument is wrong, we will come closer to understanding why change is possible.
His argument begins by taking something that does exist. Let’ s suppose that we are considering a ball. We decide to paint the ball red. The ball has changed. But the redness of ball did not exist before it was painted. So where did it come from? There are two options: it came from something that did exist or from something that did not exist. If it came from something that did exist, then the redness of the ball would have existed before the ball was painted red. Either there is really no change at all (because the ball was always red) or the redness did not come from something that exists. If it came from something that does not exist, then something can come from nothing. But that is also impossible. Therefore, Parmenides argues that change itself is impossible.
Parmenides did not consider one possibility. We consider existence to be something that is either present or absent. Something either has it or does not have it. But what if that is false. Suppose that some things partly exist and partly do not exist? If that is possible, then there would be a way out of Parmenides argument. This is precisely what Aristotle did in order to solve the argument. He also created two new words to describe the situation. When we are speaking of something that partly exists, the part that does not exist we call potential. The part that does exist we call actual. Using these things, the reality of change is made possible.
Consider a ball. The redness of the ball is a potential of the ball. It does not exist because the ball is not red. However, it could exist because the ball is the sort of thing that can be painted. Atoms do not have the potential to be red because they cannot be painted. Neither can one paint the sun. The potential that some things have exist because of something actual in those things. So let’ s consider the redness of the ball. It is actually partially actual and partially potential. It is actual insofar as the ball could be painted red, but potential insofar as the ball is not actually red. When the ball is actually being painted red, the potential of the red to be red becomes the ball actually being red. When the ball is finished being painted red, then the ball no longer has any potential to be red because it is actually red.
This distinction between actuality and potentiality can be used to explain how individuals are unified in at least one way.