It can be the studying of past ideas that have deeply influenced the cultural, societal, political and scientific norms. Something as fundamental as Cartesian epistemology characterises much of the Enlightenment and its values. It's a fundamental belief about the nature of human knowledge that has far-reaching effects. German post-Kantian idealism is the breeding ground for grand-narrative political ideologies and nationalism. Wittgenstein's analytical approach to language and his rejection of most philosophy as a language play is the basis for the post-modern worldview.