Aristotle's approach to the nature of reality is more robustly down to earth than Plato's. He accepts the need to identify something stable and enduring in a world of constant change but he rejects the notion of universal Forms or essences in the Platonic sense of items with a reality of their own distinct from particular instances of things. For Aristotle, the ultimate units of being are individual substances, for example a particular man, or a particular horse.
Aristotle arrives at this view by likening the concept of a substance with the grammatical notion of a subject. In the sentence "Bucephalus is strong, "Bucephalus", the famous warhorse of Alexander the Great, is the subject and "strong" is the predicate. We may say that strength is predicated in the subject; the quality of strength is to be found in this horse. The subject, Bucephalus, by contrast, exists in its own right: it does not have to exist in something else.
Aristotle argues that substances possess accidental or contingent properties such as being fat or healthy, or fast, or lame. These properties may change from day to day, or year to year. But substances also have according to Aristotle essential characteristics which make them the kinds of thin they are. These universal essences, for Aristotle have no independent reality of their own right. They simply exist in the particular substances of which they are instances.
While Plato puts universals higher in the order of being (for instances particular horses are but pale copies of the Form of Horse), Aristotle reverses the order. It is individual substances (like a particular horse) that exist independently; equine properties or predicates and, for example, being quadruped having, a mane, being strong, and so on, cannot exist independently, but only in a particular subject.
In short, Plato said that the essence of "horseness" is in the World of Forms not in the particular horse. Aristotle said that the essence of "horseness" is in the particular horse itself.
