The substance theory of Aristotle underlies his entire philosophy. Substance theory is the belief that substances are the ultimate things in the universe. The universe at rock bottom is not made up of elementary particles but substances. This is completely different from our modern view of the world. Aristotle defends this position in his books Categories and Metaphysics. His defense is long and detailed. Without an understanding of Aristotles logic, such a defense cannot even be understood today. Aristotle divides the world into two categories: substances and accidents- substances are the most fundamental.
A substance is a dog, a rock, a planet, a particle and a computer. An accident is something like being white, standing up, kicking that ball or being hit by Tom. It is somewhat helpful to think of substances as nouns and accidents as every other part of language. Nouns are about people, places or things. Substances are people, and things (places just refer to things). Accidents refer to features of substances. They are always the subject and object of the sentence. This gives a good indication of what substances and accidents are.
Substances are most fundamental because their existence prior to the existence of accidents and knowledge of substances is prior to that of accidents. The existence of substances is prior to that of accidents because no accident can exist unless a substance exists for it to be in. However, it is possible for a substance to exist without that accident. For example, it is possible for a desk to exist before it is painted white. Knowledge of substances is prior to that of accidents because we do not know what accidents are unless we know what substances are. We can know what substances are without knowing what accidents are. For example, we do not know what white is unless we have seen a white thing and we cannot explain whiteness without talking about white things.
We should believe substance theory for several reasons. First, it allows us to keep our common-sense understanding of the world. We can continue to believe that balls break windows, balls are things, and our standard divisions of the world are accurate. Second, it allows us to integrate a scientific understanding of the world with a common-sense understanding of it. Scientists speak of how sound waves reach our ears. If we understand that sound waves are things just as puddles are things, then we are extrapolating from what we know in order to understand what we do not know. Finally, substance theory makes nonsense of reason and science when it is rejected. If we do not believe in substances, then we are claiming that rocks, trees and computers dont exist. What really exists are atoms (or other particles) arranged in some way. Doing this is not only against common sense, but it also fails to properly explain the things we most clearly know. If we cannot do that, then no argument for particles will ever do anything but beg the question. Any argument that undercuts itself it against reason, therefore, we should all believe substance theory.