In philosophy, the classic problem of universals is the challenge of determining in virtue of what (If anything) a particular individual is a member of a category. Some philosophers, such as Armstrong, Plato, and myself believe that at least some categories are metaphysically real and hence we are seen as realists about properties. For example, if mass is a real quality, then entities with mass are grouped into that category in virtue of possessing that metaphysical property. A realist does not need to accept that all categories have a metaphysical foundation. For example, I take race and gender to be social concepts based on ideas rather than grounded in metaphysical entities. To use a less contentious example, I also take the property of being a citizen of the United States to be a social construct.
When talking about particulars being in categories, there are two general ways to view this. One is a matter of grouping—the property in question is what puts the entity in that category. There is also the matter of the entity being what it is in terms of that quality. These two can amount to the same thing, but these can be distinguished conceptually. For example, there is the matter of what it is for a green thing to be in the category of green things and the matter of what it is for an entity to be green. Again, one might determine that these amount to the same thing. To illustrate, Plato would (probably) say that the Form of Beauty groups beautiful things into that category and makes a beautiful individual beautiful. I have not, of course, gotten into the epistemic aspect of this, such as how we know that something is in a category. I have also not addressed alternatives to metaphysical realism about properties. But for the sake of what follows, I will assume properties are real so that I can get on to discussing substances and substrata.
Philosophers such as Aristotle and Descartes accepted the existence of substance and defined it as something that exists in a manner that does not depend on other entities to exist. This can get messy quick, but to keep it simple think of some everyday object like an apple. It can exist as a distinct entity, apart from other things. Yes, I know that apples come from trees and depend on the existence of dimensions such as time; but I am keeping it simple here. The ability for an apple to exist on its own is contrasted with that of its properties. To illustrate, I can buy an apple at Publix and take it home. I can take a bite out of it, taking that piece from the apple and having a mouthful of fruit. But no matter how I bite it, I cannot bite away the property of mass or shape and have a mouthful of just mass or shape. As such, properties are generally taken by many philosophers as not being substances. You can have an apple in your mouth, but you cannot have just apple shape or apple mass. Because of this sort of thinking, philosophers such as John Locke have reluctantly accepted substance, calling it “something I know not what.” Others, of course, have rejected substance. If you accept properties as existing as part of substances, there is then the matter of whether the substance is just a bundle of properties or if there is another metaphysical entity that “binds” the properties together into a substance.
While bundle theorists have the advantage of metaphysical economy, they face the problem of explaining what connects the properties together into a single entity. Substrata theorists, like me, claim that there is a second type of metaphysical entity, the substrata. It has the function of binding properties together to form objects. While I have argued in favor of substrata at length, teaching the subject again in my Metaphysics class provided me with a metaphor that might help explain the matter. Or not.
Think of properties as being like paints and objects being like paintings. For those who just believe in properties and reject substrata, a painting would just be paint. Metaphorically, one would paint a painting by painting nothing—there would just be paint touching paint, forming a painting. Naturally, one could talk about letting some paint dry and then painting on that paint like a paint canvas, but there would be the question of what the first paint was painted on.
For those who accept substrata, the canvas (or other surface) would be like the substratum and the properties would be like paint: one paints on the canvas and the canvas plus the paint forms the painting. Naturally, metaphors and analogies tend to fall apart quickly when pressed. For example, one could say that one just needs to paint on the canvas until the paint dries, then peel the paint off. While this would be a flimsy painting, a painting made entirely of paint could thus exist. At least after it had been painted on a canvas. One could also reject the paint analogy or modify it in some manner so a painting could just be paint without a canvas. But the canvas and paint metaphor has a certain appeal.