The paper considers Cassirer account of the philosophical problems raised by the theory of relativity. The main question the paper addresses is how Cassirer as a Neokantian, responds to the discoveries made by Einstein. The problem here is especially the presupposition of the a priori nature of Euclidean geometry. Cassirer answer lies in showing that Kant philosophy is broad enough to include also non-Euclidean geometries in the determination of the physical world. He does this by showing that though Kant conceived space and time as forms of pure intuition he already connected them with certain theoretical factors, with the rules of the understanding. Space as the pure form of coexistence and time as the pure form of succession imply no special relations of measurement and it is thus a mistake to assume the a priori nature of Euclidean geometry. The way different geometries can figure in the determination of the physical world is explained in reference to the Klein approach to geometry, which defines geometrical properties as those that stay invariant according to a certain group of transformations. It is the concept of a group that is the real concept a priori. Group theory plays an even larger role in physical theories as well as Cassirer epistemology. Namely, with the theory of relativity it becomes evident that physical theories are theories of invariants according to a group of transformations. Cassirer claims that the general doctrine of invariability of certain values must recur in some form in any theory of nature, because it belongs to the logical and epistemological nature of such a theory.
