An important objection to sententialist theories of attitude reports is that they cannot accommodate the principle that one cannot know that someone believes that p without knowing what it is that he believes. This paper argues that a parallel problem arises for propositionalist accounts that has gone largely unnoticed, and that, furthermore, the usual resources for the propositionalist do not afford an adequate solution. While non-standard solutions are available for the propositionalist, it turns out that there are parallel solutions that (...) are available for the sententialist. Since the difficulties raised seem to show that the mechanism by which sentential complements serve to inform us about attitudes and about sentence meaning does not depend on their referring to propositions, this casts doubt on whether talk of propositions should retain a significant theoretical role in the enterprise of understanding thought, language and communication. (shrink)