Aristotle claimed that he did not understand Plato's concept of "participation." (When a philosopher claims "not to understand" something, it means that he is pushing for a better account of it, that he is not at all satisfied so far.
Aristotle probably understood Plato as well as anybody ever has.) Aristotle's objection was, essentially, that Plato had failed to explain the relationship between the Forms and particular things, and that the word "participation" was no more than "a mere empty phrase and a poetic metaphor."
Furthermore, Plato's emphasis on the Forms made it impossible to appreciate the full reality of particular things, and the eternal permanence of the Forms made them useless for understanding how particular things could change. Indeed, the question "How do things change?" becomes the central theme of Aristotle's philosophy.
Aristotle also wanted to determine the nature of reality. But Plato had argued that reality was some-thing other than the world of our experience. Aristotle, a practical man of the earth, a great biologist, physicist, and worldly tutor to Alexander the Great, would have none of this. This world, our world, is reality. He agreed with Plato that knowledge must be universal and concerned with what things have in common, but he rejected Plato's idea that these common universal ingredients-the Iris of things-could be separated from particular things.
But this meant that Aristotle also rejected Plato's separation of the human soul from the body, and Aristotle, unlike Plato, saw human beings entirely as creatures of nature, "rational animals"-- but still animals. Metaphysics, for Aristotle, was not the study of another world, recollected in our eternal souls; metaphysics was simply the study of nature (physics), and, as importantly, the study of ourselves.
Accordingly, he brought metaphysics back home. But it must not be thought that he made it any simpler. The beginning student of Aristotle as well as the trained scholar will attest to the fact that he is among the most difficult authors in philosophy.
The doctrines of Aristotle's metaphysics sound as simple as they could be. This world, the world of our experience, is reality; there is no other world. The ultimate things of reality, which he calls substances, are individual things - men, horses, trees, and butterflies.
Change is real and reality changes (thus rejecting the chief presupposition of most who came before him). The soul is not immortal, and Forms have no existence apart from the things that are instances of them. But don't think that Aristotle thereby rejected the 250 years of Greek philosophy preceding him. Aristotle, despite the common-sense quality of his metaphysics, knew better. He was not simply returning philosophy to "common sense."
Quite the contrary, he was still answering the problems of Plato and his ancestors and making use of their answers as well. We can divide Aristotle's metaphysics into an ontology and a cosmology. For beginning philosophers, the latter is far more interesting, the former extremely difficult.
Aristotle's ontology is based upon the all-important notion of substance, his technical term for the Pre-Socratic conceptions of the ultimate "stuff" of the universe. He says, "The question that was asked long ago, is asked now, and always is a matter of difficulty, 'What is Being?' is now the question, 'What is Substance?’ "Substance," in other words, is "that which is," or reality. (It does not mean "physical material," as in "there's a gooey substance stuck to the bottom of my shoe.")
For Aristotle, the primary substances are individual things; secondary substances (less real than individuals) are what he called the species and the germs to which a thing belongs. To return to our equestrian example, this particular horse, for Aristotle, is the primary substance.
The species, "horse," and the even broader genus, "animal," are less real than...