Hans Vaihinger (1852-1933) was an important and fascinating figure in German philosophy in the early twentieth century, founding the well-known journal Kant-Studien. Yet he was overshadowed by the burgeoning movements of phenomenology and analytical philosophy, as well as hostility towards his work because of his defense of Jewish scholars in a Germany controlled by Nazism. However, it is widely acknowledged today that The Philosophy of 'As If' is a philosophical masterwork. Vaihinger argues that in the face of an overwhelmingly complex world, we produce a simpler set of ideas, or idealizations, that help us negotiate it. When cast as fictions, such ideas provide an easier and more useful way to think about certain subjects, from mathematics and physics to law and morality, than would the truth in all its complexity. Even in science, he wrote, we must proceed "as if " a material world exists independently of perceiving subjects; in behaviour, we must act "as if " ethical certainty were possible; in religion, we must believe "as if" there were a God. He also explores the role of fictions in the history of philosophy, going back to the ancient Greeks and the work of Leibniz, Adam Smith and Bentham. The Philosophy of 'As If' was a powerful influence on the emerging philosophical movement of pragmatism and was groundbreaking in its anticipation of the central role that model-building and simulation would come to play in the human sciences. This Routledge Classics edition includes a new foreword by Michael A. Rosenthal, which provides a fascinating and important background to Vaihinger's life and the legacy of The Philosophy of 'As If'.