Hume did not believe in Aristotelian substance theory. In the Treatise, he explains his reasons for rejecting it. He believes that there are two experiences that people have in regard to things. In the first, some things change. Some things stay the same through the changes that happen to them. So we see both change and sameness in the world. The substance theory of Aristotle is one way of explaining that change. Hume rejects this theory because he falsely believes that there is no evidence for the existence of substances.
Hume’s argument is quite simple. We see that the world around us changes. There are various qualities in world. Let’ s consider color and size. We might say that the ball is both red and spherical. According to Hume, we associate redness with being a sphere and call that a ball. Being both red and spherical and whatever else is true of the ball is the same as being a ball. There is nothing else there. According to Hume, Aristotelian substance theory says that there is one thing left when we subtract everything else. This one remaining thing is substance. But we cannot see substance or detect it in any way. Therefore, it does not exist.
The problem with Hume’ s argument is in two parts. The most important part is that he has misunderstood Aristotle. Aristotle believes that substances can be easily sensed and examined. The red ball is the substance. When we see the redness, we are seeing the way that the substance exists. All of the qualities are the qualities of the substance. If they were all removed then there would be no substance. So although Hume’ s argument works, he is arguing against something Aristotle never said.
The second problem with Hume’ s argument is that he rejects the real substance theory of Aristotle as well as the false theory. He believes that the ball is simply a bundle of qualities. This is absurd. Not all qualities are created equal. Some qualities are deeper than other qualities. The qualities are related to each other by a structure. If this is a rubber ball, then the quality of “being bouncy” is explained by the qualities of rubber. The red color is explained by the nature of the paint chemicals. Eventually we come to qualities so basic that the ball requires them in order to be a ball. These are the properties of the ball. The previous qualities are the accidents of the ball. The properties of the ball are all required to be a ball because they are required by what a ball is the essence of being a ball. But this is Aristotelian substance theory. Hume simply did not analyze his rejection of Aristotle nearly as deeply as he should have.