Marx considered labour as essence of humans that makes us different to other animals. But how is it more concrete than other things ? Isn't it labour situational, I mean something that forcefully to made us do ? Why not labour is an abstract concept?
For Marx (and Locke before him) labour is the one constant of human life. If we want to live we have to eat, and if we want to eat we have to do labor. There's nothing 'forced' about that, unless you consider hunger an outside force. Even in Locke's State of Nature (or the Garden of Eden for that matter) one has to get up and go pick the fruit from the trees; that's labor. Even the feudal lord, gentleman slave-owner, or fat-cat industrialist — all of whom seem to sit idly in their mansions — have to do labor. There are products to be sold, raw materials to be bought, things to be shipped, managers and muscle to hire so the peons keep in line. Even Buddhist monks have to leave their meditations each day to walk into town and beg for a bowl of rice.
- Without labor there is no production
- Without production there is no consumption
- Without consumption there is no life
Would that we lived in a world where magical faeries brought us meals every day. But then I suppose we'd just be exploiting the labor of magical faeries...
Locke held that since labor is essential for the continuation of human life, then the products of labor — the things we produce with our own efforts — should be treated as though they are part of our bodies. Thus we have a Natural Right to own, keep, and use the property we produce. Marx extended this to say that laborers — serfs, proletarians, etc.; all the low-status people who physically produce things — also have a Natural Right to the property they produce. Capitalists exploit those people (violate that Natural Right) by paying them far less than the actual value of the products of their labor, keeping the rest as profit.
In the modern (capitalist) world the concept of labor has been rendered abstract because of commodification (ala Marx). Commodification means that the process of exchanging goods and money is treated as a relation between objects, not as a relation between people. In other words, if we put down a downpayment on a house we think: "I just exchanged money for a home" (object for object). But in reality we got that money by expending someone's labor (ours or our employees'), and that house is the product of labor of a bunch of other people. So on that level we are exchanging labor for labor — a social interaction, not an object interaction — and the pertinent question would be whether the exchange is fair to all parties involved. By commodifying we obscure the social relationship, and the question of fairness disappears like a great capitalist magic trick.
TLDR: Human labor involves planning, coordination and the communication of abstract ideas as well as mastery over nature. Labor is human consciousness expressed in concrete action.
Here's a helpful passage in Marx's own words:
The animal is immediately one with its life activity. It does not distinguish itself from it. It is its life activity. Man makes his life activity itself the object of his will and of his consciousness. He has conscious life activity. It is not a determination with which he directly merges. Conscious life activity distinguishes man immediately from animal life activity. It is just because of this that he is a species-being. Or it is only because he is a species-being that he is a conscious being, i.e., that his own life is an object for him. Only because of that is his activity free activity. Estranged labor reverses the relationship, so that it is just because man is a conscious being that he makes his life activity, his essential being, a mere means to his existence.
In creating a world of objects by his personal activity, in his work upon inorganic nature, man proves himself a conscious species-being, i.e., as a being that treats the species as his own essential being, or that treats itself as a species-being. Admittedly animals also produce. They build themselves nests, dwellings, like the bees, beavers, ants, etc. But an animal only produces what it immediately needs for itself or its young. It produces one-sidedly, whilst man produces universally. It produces only under the dominion of immediate physical need, whilst man produces even when he is free from physical need and only truly produces in freedom therefrom. An animal produces only itself, whilst man reproduces the whole of nature. An animal’s product belongs immediately to its physical body, whilst man freely confronts his product. An animal forms only in accordance with the standard and the need of the species to which it belongs, whilst man knows how to produce in accordance with the standard of every species, and knows how to apply everywhere the inherent standard to the object. Man therefore also forms objects in accordance with the laws of beauty.
An import piece of context here is that Marx is reacting against Hegel's absolute idealism. Marx is a materialist. For him ideas have no essence above or outside the material world.