Returns to Cassirer’s notion of function to argue that it continuously informs his conception of philosophy. Cassirer first develops this notion in his 1910 work Substance and Function: the mathematical idea of a function connects disparate elements under a variable rule, establishing a unity through diversity (8.1). Subsequently, his early historical writings (1906‒1919) invoke the idea of a functional unity in order to explain the continuity and progress in the history of thought while simultaneously acknowledging the legitimacy of each historical epoch (8.2). Cassirer’s mature, systematic, writings (1923‒1942) reconcile the unity of human culture and the synchronic diversity of our cultural domains by means of the same idea (8.3). Finally, the ethical reflections that we find in Cassirer’s latest writings (1935‒1946) are also rooted in this ‘functional conception of philosophy’ (8.4).