His conception of history was undergoing a transformation in a direction similar to that towards which his conception of philosophy was moving. When he passed from the consideration of the general theory to the examination of more technical aspects of Marx's doctrines, the first difficulty which presented itself to him was that of the relations between Marxian economics and pure economics, or general economic science. The society whose economic life Marx had studied in _Das Kapital,_ was neither human society in general, nor any particular historical society, but a purely ideal and formal society deduced from a proposition assumed outside the fields of pure economics: that of the equivalence of value and labour. Starting from this postulate, Marx had proceeded to inquire into those processes of differentiation between the assumed standard and the actual prices of commodities in a capitalist society, by which labour itself acquires a price and becomes a commodity. It was a method of scientific analysis consisting in regarding a phenomenon not as it actually exists, but as it would be if one of its factors were altered, and in comparing the hypothetical with the real phenomenon, conceiving of the first as diverging from the second which is postulated as fundamental, or the second as diverging from the first, which is postulated in the same manner. It is only when the whole of Marxian economics is considered as the application of such a method, that the concepts of labour-value and of surplus-value acquire a definite and precise meaning: the description of economic society as a pure working society (producing no goods which cannot be increased by labour) must then be interpreted as a concept of difference, or an instrument of elliptical comparison, as against the descriptions of actual economic society given by pure economics. Its positive value, not as an abstract hypothesis, but as a means of knowledge, depends on the fact that such a society does actually coincide with certain aspects of historical capitalist society; that the equivalence of labour and value is not a purely imaginary fact, but a fact among other facts, empirically opposed, limited, and distorted by other facts. Having assumed this equivalence as a test for the study of the social problem of labour, Marx's object was to show the special way in which this problem is solved in a capitalist society. And this was the real justification for his employment of the hypothetical method.