Keith and I are definitively confused regarding the meaning of "abstraction".
Like many others we're treating "abstract" and "concrete" as synonyms for "mental" and "material". Where two sources of muddle operate simultaneously.
First, phrases like "abstract thought" suggest an equivalence by placing the words next to each other. This is logically incoherent, but the juxtaposition appears to imply relationship. This is indeed the dictionary definition of "abstract" as an adjective: "existing in thought or as an idea but not having a physical or concrete existence". But this is not helping, at all, with the logic of abstraction in Capital.
Second, the word "concrete" is the same word used to indicate the stuff sidewalks are made of, which is pretty fukkin' material, just ask anyone. Note its use this way in the dictionary definition above. In both cases we're misled by words which appear to point in unhelpful directions.
The synonym we need for "abstract" is simple, while for "concrete" it's complex. To abstract means to reduce complexity by focusing on smaller groups of determinants, or variables, than are naturally present. For example by focusing on interactions of supply and demand to the exclusion of availability of substitutes and so on. The dictionary has this right when considering "abstract" as a verb: "consider (something) theoretically or separately from something else". By abstracting you remove some number of determinants from their full context in order to consider them in isolation. Thus Marx's logic of exposition in Capital, by proceeding from abstract to concrete, is not a movement from thought to the real. It's a movement from simple to complex which always remains within thought itself.
Keith and I are very much not getting there on our own. We're undergrads at a little speck of a school which lacks the resources to help us with questions like this. Tonight we've isolated ourselves further by abstracting our presences from human company — see what I did there? We're alone in the Coz McMuffin, a pint-sized coffee-house-slash-speakeasy in one corner of the University's Dwayne W. Orton student center, an ugly rectangular concrete building — there, I did that thing again — like the inside of a very drab cake box, with a pile of dictionaries, the Vintage edition of Capital, the stupid Cold War Marx/Engels anthology edited by Tucker, and the McLellan bio of Marx, where our confusion organizes and bullies all of these sources, a nifty example of Althusser's structure-in-dominance although we haven't yet found Althusser. We very much want to sort this question, but without expert help we're trapped in our little infinite loop of "but that means... which implies..."
Absent from this picture is our Philosophy professor. In a rational universe he'd be the resource to whom to turn. Yet we know from experience he won't help. It's unclear from his ironic pose whether he doesn't know the answers, or doesn't think we're worth bothering with, or dislikes us personally, or is too drunk to follow the questions, or if we have one too many varied chromosomes to be worth his attention. Doesn't matter. We don't turn to him, because it's pointless, because he'll brush us off with a joke and a smirk. We know this because that's what he does.
So we thrash. As the evening grows old and our brain cells retire without us we use my master key to sneak into the kitchen, where Keith cooks us a late meal of rice and stir-fry. We don't leave a mess — Keith cooked so I clean — but we do raid the ice cream supply. As dawn approaches we realize we've got class in a few hours. We've gotten nowhere, but we're trying, and I'm happy there's at least one person in the picture who's devoted to trying with me.