Reichenbach articulated and defended a distinction between conventional and factual components within a scientific theory. For example, he argued that the axioms of geometry are factual while the coordinative definitions for geometry are conventional.1 The scientific spirit with which Reichenbach approached philosophical discussion is illustrated by the manner in which he clarified his doctrine when confronted with criticism. His reply to Einstein represents, I shall argue, a fundamental discovery that refutes both the conventionalism of Poincare and Quine’s claim that no categorical distinction can be drawn between those statements that are true as a matter of fact and those that are true by convention. I shall conclude with a comparison between Reichenbach’s doctrine and a similar one proposed by Carnap.