Two innaresting paragraph’s from Althusser’s preface to Capital:
‘scientific abstraction is not at all ‘abstract’, quite the contrary. E.g., when Marx speaks of the total social capital, no one can ‘touch it with- his hands’ ; when Marx speaks of the ‘total surplus-value’, no one can touch it with his hands or count it: and yet these two abstract concepts designate actually existing realities. What makes abstraction scientific is precisely the fact that it designates a concrete reality which certainly exists but which it is impossible to ‘touch with one’s hands’ or ‘see with one’s eyes’. Every abstract concept therefore provides knowledge of a reality whose existence it reveals : an ‘abstract concept’ then means a formula which is apparently abstract but really terribly concrete, because ofthe object it designates. This object is terribly concrete in that it is infinitely more concrete, more effective than the objects one can ‘touch with one’s hands’ or ‘see with one’s eyes’ and yet one cannot touch it with one’s hands or see it with one’s eyes. Thus the concept of exchange value, the concept of the total social capital,the concept of socially necessary labour, etc. All this is easy to explain.
The second point: the basic concepts exist in the form of a system, and that is what makes them a theory. A theory is indeed a rigorous system of basic scientific concepts. In a scientific theory, the basic concepts do not exist in any given order, but in a rigorous order. It is therefore necessary to know this order, and to learn the practice of rigour step by step. Rigour (systematic rigour) is not a fantasy, nor is it a formal luxury, but a vital necessity for all science, for every scientific practice. It is what Marx in his ‘Afterword’ calls the rigour of the ‘method of presentation’ of a scientific theory’. (Lenin and Philosophy and Other Essays 76-7)