By rejecting the essence of man as his theoretical basis, Marx rejected the whole of this organic system of postulates. He drove the philosophical categories of the subject, of empiricism, of the ideal essence, etc., from all the domains in which they had been supreme. Not only from political economy (rejection of the myth of homo economicus, that is, of the individual with definite faculties and needs as the subject of the classical economy); not just from history (rejection of social atomism and ethico-political idealism); not just from ethics (rejection of the Kantian ethical idea); but also from philosophy itself: for Marx’s materialism excludes the empiricism of the subject (and its inverse: the transcendental subject) and the idealism of the concept (and its inverse: the empiricism of the concept).
This total theoretical revolution was only empowered to reject the old concepts because it replaced them by new concepts. In fact Marx established a new problematic, a new systematic way of asking questions of the world, new principles and a new method. This discovery is immediately contained in the theory of historical materialism, in which Marx did not only propose a new theory of the history of societies, but at the same time implicitly, but necessarily, a new ‘philosophy’, infinite in its implications. Thus, when Marx replaced the old couple individuals/human essence in the theory of history by new concepts (forces of production, relations of production, etc.), he was, in fact, simultaneously proposing a new conception of ‘philosophy’. He replaced the old postulates (empiricism/idealism of the subject, empiricism/idealism of the essence) which were the basis not only for idealism but also for pre-Marxist materialism, by a historico-dialectical materialism of praxis: that is, by a theory of the different specific levels of human practice (economic practice, political practice, ideological practice, scientific practice) in their characteristic articulations, based on the specific articulations of the unity of human society.
In a word, Marx substituted for the ‘ideological’ and universal concept of Feuerbachian ‘practice’ a concrete conception of the specific differences that enables us to situate each particular practice in the specific differences of the social structure.