From the April 1980 issue of the Socialist Standard
Take the theory of water motion. We pipe water, we regulate its flow differently in ordinary wells, in artesians, springs, etc. Is the theory of the flow of water explained by listing the specific bores, drills, pumps, and pipes? No. There is an abstract physical theory of the flow of all water, the science of hydraulics, and this abstract theory ignores the individual forms of the motion of water, describes no particular form of water whatever, and describes no actual phenomenon exactly as it takes place. But its theory describes them and its laws govern them and unless we have this abstract theory we have no means of understanding anything. And this despite the fact that water is abstractly described and yet, in practice, it is always concretely availed of.
We ask how and where does profit come from? From surplus-value. And that? From labour-time. And how does it flow? Through pipings composed of constant and variable capital. And how is it sprinkled or flushed throughout the economic system? As an average rate of profit. But the law of the composition and flow of value, like that of the composition and flow of water, is the same throughout. If you cannot correlate all the forms of conducting and utilising water exactly in conformity with the governing theory of hydraulics, that unfortunately is the nature of the world we live in and no one can transcend it, not even Böhm-Bawerk.
That no science is capable of application to every permutation and combination of circumstance, under the exact application of theory, is the weakness of man's mind, of his perception of the world. Is that a reason for repudiating science? Then all political economy (and all knowledge) is equally threatened. We abstract from a hundred appearances to get one common explicatory factor.[from Elements of Marxian Economic Theory and its Criticism by William J. Blake]