  Conventionalism about some phenomenon is the doctrine that, perhaps despite appearances to the contrary, the phenomenon arises from or is determined by convention. Conventionalism surfaces in virtually every area of philosophy, with respect to such topics as property (Hume’s Treatise of Human Nature), justice (Hume’s Treatise again), morality (Gilbert Harman (1996), Graham Oddie (1999), Bruno Verbeek (2008)), geometry (Henri Poincaré (1902), Hans Reichenbach (1922), Adolf Grünbaum (1962)), Lawrence Sklar (1977)), pictorial representation (Nelson Goodman (1976)), personal identity (Derek Parfit (1984)), ontology (Rudolf Carnap (1937), Nelson Goodman (1978), Hilary Putnam (1987)), arithmetic and mathematical analysis (Rudolf Carnap (1937)), necessity (A. J. Ayer (1936)), Alan Sidelle (1989)), and almost any other topic one can imagine. Conventionalism arises in so many different forms that one can say little of substance about it as a general matter. However, a distinctive thesis shared by most conventionalist theories is that there exist alternative conventions that are in some sense equally good. Our choice of a convention from among alternatives is undetermined by the nature of things, by general rational considerations, or by universal features of human physiology, perception, or cognition. This element of free choice distinguishes conventionalism from doctrines such as projectivism, transcendental idealism, and constructivism about mathematics, all of which hold that, in one way or another, certain phenomena are “due to us.”