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(5) John knows how to play a piano sonata implies
(6) John has the ability to play a piano sonata.

It is important to distinguish between sentences and propositions. Paul would say, ‘‘The sky is blue,’’ and Pierre would say, ‘‘Le ciel est bleu.’’ Though each expresses his belief by a different sentence, each believes the same proposition.

Propositional knowledge is knowledge of facts or true propositions. So, consider the following examples of propositional knowledge:
(1) John knows that Caesar was assassinated. (2) John knows that the sky is blue.
In these examples, the objects of knowledge, or what is known, are, respectively, the true propositions that Caesar was assassinated and that the sky is blue.