It is a heterodox school of political economic thought. Its foundations can be traced back to the critique of classical political economy in the works of Karl Marx and Friedrich Engels. Marxian economics comprises several different theories and includes multiple schools of thought which are sometimes opposed to each other. In many cases, Marxian analysis is used to complement or supplement other economic approaches. As one does not necessarily have to be politically Marxist to be economically Marxian, the two adjectives coexist in usage rather than being synonymous in nature. They share a semantic field while also allowing connotative and denotative differences. Marxian economics concerns itself variously with the analysis of the crisis in capitalism, the role and distribution of the surplus product and value in various types of economic systems, the nature and origin of economic value, the impact of class and class struggle on economic and political processes, and the process of economic evolution.