Karl Marx: “The philosophers have only interpreted the world, in various ways. The point, however, is to change it.” (11th Thesis on Feuerbach, and the epitaph on Marx's grave)
Ludwig Wittgenstein: “What is the use of studying philosophy if all that it does for you is to enable you to talk with some plausibility about some abstruse questions of logic, etc., & if it does not improve your thinking about the important questions of everyday life?” (Letter to Norman Malcolm, November 16, 1944)