Truth and Other Enigmas

Frege held that truth and falsity are the references of sentences.

Sentences cannot stand for propositions (what Frege calls 'thoughts'), since the reference of a complex expression depends only on the reference of its parts; whereas if we substitute for a singular term occurring in a sentence another singular term with the same reference but a different sense, the sense of the whole sentence, i.e. the thought which it expresses, changes.