Cassirer’s philosophy of symbolic forms offered the theoretical framework for a unified study of culture, including such nonrational forms assumed by human understanding of the world as mythical thinking. At the same time, Cassirer defended the role of mathematics and natural science as models of rationality in the Kantian sense. This paper offers a discussion of the role of mathematics in Cassirer’s philosophical project, given the fact that he first developed the notion of symbolic form in order to account for the rationality of theory change in physics. The historical perspective of Cassirer’s approach suggests that rationality depends not so much on the assumptions of some specific theory as on the unifying power of mathematical method. He argued for a model of rational thinking which, owing to its symbolic character, can be articulated in various ways without being contradicted by the fact that there are different symbolic forms.