The central doctrine of empiricism - that true knowledge or understanding of the world comes ultimately from sense impressions - has underpinned most of the practices and arguments of historians for the past 150 years. Empirical history has always faced attacks from a variety of other approaches, however, including Marxism, philosophical idealism and, most recently, postmodernism. In spite of this, empirical methods have prevailed, and the study of history remains broadly empirical.