There is no need for us to go into detail here about the nature and function of these categories. In applying his realism to particular cases Aristotle used a teleological method. That is, he argued that a thing's form can best be discovered by inquiring about its purpose. Such questions will help us to explain why a particular piece of matter has the particular form it does. Aristotle used his teleological method as an integral part of his task of classifying numerous natural and intellectual objects.
Modern science is, of course, one of the fruits of the empiricist tradition.
So it should come as no surprise to find that many of the names we now give to the different branches of the sciences, as well as other academic disciples, were first established as such by Aristotle's teleological classifications. Let me explain the epistemological difference between Aristotle's realism and Plato's idealism by using this piece of chalk as an example. This piece of matter we call chalk is real only because it participates in a real idea, the idea of chalkness. This means the matter must be able to fulfill the purpose of chalk. Obviously, when it appears in a classroom, at least, chalk is used to write things on the blackboard.