But, even when these two points are settled, the real essence of Marx's investigation is not yet explained. Were _Das Kapital_ nothing but what we have so far defined, it would be merely an _economic monograph on the laws of capitalist society_.[13] Such a monograph Marx could only have made in one way: by deciding on these laws, and explaining them by general laws, or by the fundamental concepts of economics; by reducing, in short, the complex to the simple, or passing, by deductive reasoning, and with the addition of fresh hypotheses, from the simple to the complex. He would thus have shown, by precise exposition, how the apparently most diverse facts of the economic world are ultimately governed by one and the same law; or, what is the same thing, how this law is differently refracted as it takes effect through different organisations, without changing itself, since otherwise the means and indeed the test of the explanation would be lacking. Work of this nature had been already carried out, to a great extent, in Marx's time, and since then it has been developed yet further by economists, and has attained a high degree of perfection, as may be seen, for instance, in the economic treatises of our Italian writers, Pantaleoni and Pareto. But I much doubt whether Marx would have become an economist in order to devote himself to a species of research of almost solely theoretical, or even scholastic, interest. His whole personality as a practical man and a revolutionist, impatient of abstract investigation which had no close connection with the interests of actual life, would have recoiled from such a course. If _Das Kapital_ was to have been merely an economic monograph, it would be safe to wager that it would never have come into existence. What then did Marx accomplish, and to what treatment did he subject the phenomena of capitalist society, if not to that of pure economic theory? _Marx assumed, outside the field of pure economic theory, a proposition; the famous equivalence between value and labour; i.e. the proposition that the value of the commodities produced by labour is equal to the quantity of labour socially necessary to produce them._ It is only with this assumption that his special investigation begins. But what connection has this proposition with the laws of capitalist society? or what part does it play in the investigation? This Marx never explicitly states; and it is on this point that the greatest confusions have arisen, and that the interpreters and critics have been most at a loss.