Marxism is a category theory. That means it’s a theory that seeks to uncover the basic categories of a phenomenon and ground them and show their origin.
The way I’m using it, a category is a metaconcept. It’s a concept that shows you how to use other concepts. To illustrate, let’s say you have two concepts that aren’t categories, like “table” and “brown”. There are many ways you can assemble these concepts in judgments, but only some of these judgments will make logical sense, and only a portion of the ones that make logical sense will actually be about the world. So you can say, “The brown is table.” This is nonsense. But you can say, “The table is brown,” which makes sense. The reason it makes sense is because the concept “table” occupies the “subject” position of the sentence, and the concept “brown” occupies the predicate position. Perhaps “brown” could occupy a subject position in a judgment, for example if you’re talking about the properties of the color brown. But “table” can never occupy the predicate position, because a table is never a property of something else. The reason we recognize this difference is because of categories. We know that a table is the sort of thing that is only a substance and never an accident. To judge that something is a substance or an accident is to make a categorical judgment about those things. Take another example: ice melting when a flame is applied to it. Categories allow us to understand that heat is the cause of the ice melting, not that the ice melting causes the heat. We place the heat in the “cause” position, and we place the melting in the “effect” position. We apply categories to those things, and by doing so, we know how to think about them, how to say things about them, and so how to know about them.
Marxism is a category theory because it attempts to come up with the most fundamental concepts applicable to the anatomy of civil society (political economy). Perhaps the most important category Marx discovers is “surplus-value”, which is the fulcrum around which his theory of the “valorization of value” turns. Understanding what surplus-value is (and the labor that produces it, surplus-labor) allows you to order the other phenomena of capitalist society in such a way as to make sense of the actual world, rather than to deal with mere subjective appearances or nonsense.
Part of the problem with technocratic explanations of capitalism and even technology is that they use concepts like “cognitive surplus” or “creative class” not only without knowing what they mean but without any fundamental understanding of what capitalism is. This leads to a picture of the world which is inverted. Claiming that technology on its own brings about a revolution or that high technology capitalism is capable of smoothing over the class conflict is like saying that the melting of ice causes a flame to ignite. It’s not that technocrats observe different phenomena from us. They see the same revolutionary transformations of society we do. But they order the phenomena of experience in such a way that substance becomes accident and vice versa. They remain purely at the level of appearance and never penetrate to the essence of the thing.
Every category theory is faced with the problem of explaining where the categories come from. Kant famously attempted to ground the table of categories in the table of logical judgments—in other words to make transcendental logic project a kind of transcendental syntax on to the world. This is not very different from more modern attempts by philosophers to ground metaphysics in the philosophy of language, thus making linguistic analysis first philosophy.
Marx does not attempt to ground the categories of political economy in thought, language, or anything of the sort. Instead, he grounds the categories of political economy in the most fundamental social relations composing civil society: the capital social relation. Yet the capital social relation itself is historical: it came into existence at a particular point in time out of other social relations. For this reason, Marx believes the categories of political economy are historical in nature, i.e., they came into being at a certain point, and they may very well pass away again, too. But because these social relations are themselves concrete relationships between people, based in the way the society produces and reproduces itself, Marx’s theory is rightly called “materialist”.
Therefore, Marx’s category theory arises by means of a critique of political economy, and the theory Marx uses to explain the ground and origin of these categories is historical materialism.