Without defining "ideology", Marx used the term to describe the production of images of social reality. According to Engels:is a process accomplished by the so-called thinker consciously, it is true, but with a false consciousness. The real motive forces impelling him remain unknown to him; otherwise it simply would not be an ideological process. Hence he imagines false or seeming motive forces. Because the ruling class controls the society's means of production, the superstructure of society (i.e. the ruling social ideas), are determined by the best interests of the ruling class. In "The German Ideology", Marx says that "[the ideas of the ruling class are in every epoch the ruling ideas, i.e. the class which is the ruling material force of society, is, at the same time, its ruling intellectual force". The term "political economy" initially referred to the study of the material conditions of economic production in the capitalist system. In Marxism, political economy is the study of the means of production, specifically of capital and how that manifests as economic activity.