The termsubstancedesignates those things that are most fundamental to existence. However, since there is no clear or definite answer as to what those things are, substanceis effectively a metaphysical placeholder, a word that refers to a problem rather than a definable thing. Aristotle points out that some things do seem to be more fundamental than others. For example, colors can only exist if there are physical objects that are colored, though it seems conceivable that physical objects could potentially exist in a world devoid of color. If there is a hierarchy to being, such that some things are more fundamental than others, there must be a most fundamental thing on which everything else depends. Aristotle thinks that he can approach this most fundamental thing by examining definition. Properly speaking, a definition should list just those items without which the thing defined could not exist as it is. For instance, the definition of a toe should mention a foot, because without feet, toes could not exist. Since we cannot define toes without making mention of feet, we can infer that feet are more fundamental than toes. A substance, then, is something whose definition does not rely on the existence of other things besides it.
Aristotle’s insistence on the primacy of substance reflects his view that there is no single category of being. We can talk about existence in connection with all sorts of things. Colors exist, ideas exist, places exist, times exist, movements exist, and so on. Part of Aristotle’s insight is that these things do not all exist in the same way. That is, there is not some one thing called “existence” in which colors and places partake in markedly different ways. Rather, there are different categories of existence that apply to different categories of things. Colors and places have two entirely different kinds of existences. However, if different sorts of things exist in different ways, how is it that there seems to be a single cosmos in which color, place, time, and all the rest, seem to exist together? The fact that color and substance have two different kinds of existence does not prevent substances from being colored. For the cosmos to be unified, there must be a base unit of existence on which all other kinds of existence depend. Aristotle’s argument for the primacy of substance, then, is his way of saying that it is substance, and not time or location, that binds the cosmos together.