Aristotle`s metaphysics is based on Plato’s. Parmenides thought that change was impossible because it would require something coming into existence from nothing. Heraclitus on the other hand, argued that there is nothing but change, there is nothing that is constant. Both were brilliant philosophers. However, Plato’s genius was that he was able to reconcile these two views. He thought we lived in 2 worlds, the lower world and the higher world. Or, the world of particular things, and the world of the forms (or universals). In the world of particular things, things can change, but in the world of forms nothing changes. Plato believed that the forms of things, like universals actually exist in their own realm so to speak. Thus, he believed things like numbers actually exist. He believed for instance perfect justice existed, perfect love, the perfect circle all these things existed as not just abstractions, but really existed in the forms. But, not in particular things. For nothing could be the perfect circle for instance. As all particular circles are not perfect. And the perfect circle can have no line thickness. Since the dots that make the line are at varying radii from the center.
Aristotle was more of a realist. He did not see all universals as actually existing, but as abstractions, existing in the things themselves, and in the mind of the person thinking about them. Aristotle taught that everything is composed of form and matter. And that God was perfect form, pure actuality, with no potentiality, and thus immutable.
Aquinas took Aristotle and came up with his 5 ways or proofs for God’s existence.
Plato belived that we can acquire knowledge not just empirically (through the senses) but that we are all born with certain knowledge that can become known to us. Knowledge from the realm of the forms, where he believes we all come from. Knowledge that we forgot when we became born. Whereas, Aristotle and Aquinas were more empiricists, believing all knowledge comes to us through the senses. What is interesting is that Aquinas actually got criticized for putting so much weight on Aristotle’s theories at a time when most Christian philosophers were Platonists.