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Sometimes knowledge enters philosophical consideration in the form of a propositional attitude. The SEP says it well:

Propositional attitude reporting sentences concern cognitive relations people bear to propositions. A paradigm example is the sentence ‘Jill believes that Jack broke his crown’. Arguably, ‘believes, ‘hopes’, and ‘knows’ are propositional attitude verb and, when followed by a clause that includes a full sentence expressing a proposition (a that-clause) form propositional attitude reporting sentences. Attributions of cognitive relations to propositions can also take other forms. For example, ‘Jack believes what Jill said’ and ‘Jack believes everything Jill believes’ are both propositional attitude ascriptions, even though the attitude verb is not followed by a that-clause. Some philosophers and linguists also claim that sentences like ‘Jill wanted Jack to fall’, ‘Jack and Jill are seeking water’, and ‘Jack fears Jill’, for example, are to be analyzed as propositional attitude ascribing sentences, the first saying, perhaps, something to the effect that Jill wants that Jack falls, the second that Jack and Jill strive that they find water, and the third that Jack fears that Jill will hurt him. But such analyses are controversial. (See the entry on intensional transitive verbs.)

Having a successful theory of propositional attitude reports is important, as they serve as a converging point for a number of different fields, including philosophy of language, natural language semantics, philosophy of mind, metaphysics, and epistemology.

In this article, we examine attempts to deal with a puzzle about propositional attitude reporting sentences that was first posed by Gottlob Frege in his 1892. Subsequent literature has been concerned with developing a semantic theory that offers an adequate treatment of this puzzle. We present the main theories and note the considerations that count in their favor and some of the problems that they face.

1. Frege’s puzzle — sep article on propositional attitude reporting

I recently did a thread in Frege's puzzle, but I'd be happy to revisit it if anyone's interested.