This paper shows that Cassirer’s philosophy of mathematics underwent a significant transformation by the end of the 1920s. This transformation was due to Cassirer’s reception of the ‘foundational crisis’ within mathematics itself. David Hilbert’s conception of the ‘ideal elements’ of mathematics attracted Cassirer’s particular attention. Indeed, he sought a ‘transcendental deduction’ of these elements. Reflection on this issue is therefore essential to providing an adequate interpretation of the later Cassirer’s enterprise in the philosophy of mathematics.