  Hans Vaihinger (1852–1933) was a Neo-Kantian philosopher, who made important contributions to epistemology, philosophy of science and mathematics, and to the historiography of philosophy. As a historian, Vaihinger produced groundbreaking work on Kant’s theoretical philosophy, especially on the Transcendental Aesthetic and Transcendental Deduction of the Categories, as well as one of the first serious philosophical commentaries on Nietzsche. As a systematic philosopher, Vaihinger is best remembered as the father of the philosophical theory of fictionalism, which he sets out in his magnum opus Die Philosophie des Als Ob (PAO), which translates to The Philosophy of As-If. Heavily influenced by the nineteenth-century Neo-Kantian Friedrich Albert Lange, as well as by Arthur Schopenhauer, the PAO develops a position that Vaihinger calls “idealistic positivism.” The view seeks to wed elements of Kant’s transcendental idealism with the Darwinian theory of natural selection, and places central emphasis on the epistemic import of idealization and fiction. Vaihinger’s fictionalism anticipated many of the anti-realist and anti-metaphysical strains of later logical positivism, and represented an important alternative to the Neo-Kantianism of the Marburg, Southwest, and Göttingen Schools.