It is difficult to draw a line between the ideological and the scientific element in economic theory. The subject-matter here is the very essence of the egoistic struggle of men, a preoccupation of their unconscious natures. And the science is of necessity abstract and indefinite. It is a matter of formulating large groups of facts, rather than accurately defining particulars, and in this act of formulation the hidden purpose of the scientist has a very free hand. However, if an economist who acknowledged a revolutionary purpose, and one who acknowledged a conservative purpose, sat down to formulate the laws upon which they could agree, we should still have something of an objective science of economics. And it is only by conceiving this science as a thing apart from the expressions of class-interest which have been bound up in it, that we can adequately describe the economic contributions of Karl Marx. He contributed certain fundamental and enduring ideas or attitudes to the science of economics, and he replaced the business man’s ideology that is usually bound up in that science with a proletarian ideology.