In order to understand what postmodernism is about it is essential to understand what modernity means for the social sciences and this is linked to what is deemed to be the enlightenment project. Modernity or the enlightenment project is reflected in positivism, the importance of the scientific method. The age of enlightenment ushered in human rationality as the source of knowledge, thus encouraging the rejection of previous authorities such as the church or custom. Enlightenment undermined the authority of the monarchy and paved the way for the political revolutions of the 18th and 19th centuries. A variety of 19th-century movements, including liberalism and neo-classicism, trace their intellectual heritage to the Enlightenment.
This lead to the belief that the social sciences would extend this enlightenment project into explanations of the collective solidarity, collective activities and relationships of human beings. These beliefs shape social sciences by giving science a privileged position in the pursuit of truth, encouraging the search for sets of concepts to provide a framework for understanding social life regardless of particular social situations or time and the acceptance of metanarratives as superior to other narrative accounts about society. The political philosopher Montesquieu introduced the idea of a separation of powers in a government, a concept which was adopted by the authors of the United States Constitution.
Enlightenment project is apparent in some of the works of Karl Marx. Marxian theory is a large metanarrative about the historical development of western societies such that it includes all stories about society and because of its claim to be based on scientific observation and its use of a conceptual framework like modes of production and social relations of production, it claims a privileged position and a universal nature.