In this paper, I will focus on Ernst Cassirer’s account of hypotheses in order to argue that: (1) Cassirer provides for two distinct concepts of hypotheses—the ontological and the operational concept of hypotheses; (2) these two concepts are based on two distinct views on how a hypothesis is introduced and developed in the available theory and, therefore, depend on a distinction in the epistemological theory; and (3) this account helps to explain what role the hypothesis of ether had in Albert Einstein’s development of the theory of special relativity and also gives systematicity to Einstein’s treatment of the idea of ether.