Sir Karl Raimund Popper was an Austrian and British philosopher and a professor at the London School of Economics. He is counted among the most influential philosophers of science of the 20th century, and also wrote extensively on social and political philosophy. Popper is known for repudiating the classical observationalist/inductivist account of scientific method by advancing empirical falsification instead; for his opposition to the classical justificationist account of knowledge which he replaced with critical rationalism, "the first non justificational philosophy of criticism in the history of philosophy" and for his vigorous defense of liberal democracy and the principles of social criticism which he took to make the flourishing of the "open society" possible.