The problems which are discussed by Frege in "On Sense and Reference" and to which he proposes a solution in that famous paper might seem to be of interest primarily to philosophers of language and to logicians. It is not hard to see, however, that they are also of an intensive interest to epistemologists. They are highly relevant to a problem which has not (it seems to me) received its fair share of epistemologists' attention. This is the problem of the objects of knowledge. Moreover, both Frege's discussion and my criticism of it can be extended to other epistemologically important concepts, including belief, memory, and thinking.