Abstraction is not a return of thought into itself in order
to grasp its real essence (the rational subject): it is rather an exclusively
experimental procedure of investigation of the concrete (historically
determinate social relations). It follows that concepts are not introduced once
and for all at a single level of abstraction. They are transformed by the
characteristic interplay which constitutes the passage from the abstract to the
concrete and enables the concrete to be absorbed within theory.
(Michel Aglietta: A Theory of Capitalist Regulation. The
US experience; London: NLB, 1979: 15)
It seems to be correct to begin with the real and the concrete, with the real precondition, thus to begin, in economics, with e.g. the population, which is the foundation and the subject of the entire social act of production. However, on closer examination this proves false. The population is an abstraction if I leave out, for example, the classes of which it is composed. These classes in turn are an empty phrase if I am not familiar with the elements on which they rest. E.g. wage labour, capital, etc. These latter in turn presuppose exchange, division of labour, prices, etc. For example, capital is nothing without wage labour, without value, money, price etc. Thus, if I were to begin with the population, this would be a chaotic conception [Vorstellung] of the whole, and I would then, by means of further determination, move analytically towards ever more simple concepts [Begriff], from the imagined concrete towards ever thinner abstractions until I had arrived at the simplest determinations. From there the journey would have to be retraced until I had finally arrived at the population again, but this time not as the chaotic conception of a whole, but as a rich totality of many determinations and relations. The former is the path historically followed by economics at the time of its origins. The economists of the seventeenth century, e.g., always begin with the living whole, with population, nation, state, several states, etc.; but they always conclude by discovering through analysis a small number of determinant, abstract, general relations such as division of labour, money, value, etc. As soon as these individual moments had been more or less firmly established and abstracted, there began the economic systems, which ascended from the simple relations, such as labour, division of labour, need, exchange value, to the level of the state, exchange between nations and the world market. The latter is obviously the scientifically correct method. The concrete is concrete because it is the concentration of many determinations, hence unity of the diverse. It appears in the process of thinking, therefore, as a process of concentration, as a result, not as a point of departure, even though it is the point of departure in reality and hence also the point of departure for observation [Anschauung] and conception.
(From: Karl Marx's Outline of the Critique of Political
Economy [Grundrisse] - http://www.marxists.org/archive/marx/works/1857/grundrisse/ch01.htm