It is a well-known fact that Ernst Cassirer was inspired by his colleague, the biologist Jakob von Uexküll at the university of Hamburg. This paper claims this inspiration was double—affecting both Cassirer’s philosophical anthropology and Cassirer’s epistemology of biology, but in two rather different ways. Thus, the paper intends to shed light on a corner of the history of the development of German thought of the interwar period. It may also have an actual interest because both Cassirer and Uexküll enjoy, for the time being and each in their way, a renaissance, e.g. in the recent field of biosemiotics.