Marx makes a lot of how others ‘abstract’ concepts. This is something I’ve been struggling with in Marx, so I found this article, which does a good job of explaining how Marx arrived at his approach to abstraction, and why he feels everyone else up to him was getting it backwards.

Taken from this article:
Power and Necessity of Abstraction in Marx  
by Robert Nigro

The question of abstraction plays a crucial role in Marx’s work. Marx was a materialist and his approach was a critique of universals as we know them. Marx analyzed and criticized the historical conditions that allowed the discourses to assume validity and truth. His materialism challenged the idealism of the 18th century. His claim is that the notion of abstraction has been distorted and mystified by philosophers and classical political economists. Marx draws on the notion of determinate abstraction.

Marx says “it seems to be correct to begin with the real and concrete. So for example, in economics, to start with the population, which is the foundation and subject of the social act of production. But this is false.” Starting from the real and concrete to derive an abstract is scientifically incorrect. Marx proposes taking the concrete as the result. The concrete is concrete because it has encompassed all the factors and is a unity of the diverse.

The correct method is to go from the abstract to the concrete to the determination. Determination is the product of a theoretical approximation, which utilizes general abstractions, polarities, and dimensions for this end.

Truth is an objective for the analysis, not its starting point. He outlines a path of abstract knowledge, rising from the simple to the combined, that helps us to discover, to invent reality. Instead of just reproducing reality, abstraction is the thinking process that leads to grasp antagonisms, and contradictions crossing reality.  

But invention doesn’t mean idealism. For Marx, abstraction is in no way the search for essences or eternal truths or universals. Marx doesn’t accept such things. Individuals don’t have a ‘human nature’, some form or substance that fills each. For Marx, there is only the relation- what happens among individuals. Abstraction must grasp the multiple and active relations individuals establish with each other. It is these relations that define what individuals have in common. But this common is not a pre-existing thing, what the abstraction would bring to language.  

The importance of this approach is to wonder what happens if we don’t accept a priori the existence of ready-made notions. For Marx, the truth is the result of practices emerging from struggles and social relations. Working from those practices and from understanding the forces at work, analysis must be able to tell in advance what possible forms the development of historical and social processes might assume.