Marxist philosophy or Marxist theory are works in philosophy that are strongly influenced by Karl Marx's materialist approach to theory, or works written by Marxists. Marxist philosophy may be broadly divided into Western Marxism, which drew out of various sources, and the official philosophy in the Soviet Union, which enforced a rigid reading of Marx called dialectical materialism, in particular during the 1930s. Marxist philosophy is not a strictly defined sub-field of philosophy, because the diverse influence of Marxist theory has extended into fields as varied as aesthetics, ethics, ontology, epistemology, theoretical psychology and philosophy of science, as well as its obvious influence on political philosophy and the philosophy of history. The key characteristics of Marxism in philosophy are its materialism and its commitment to political practice as the end goal of all thought.