Frege on Compound Thoughts: A Wittgensteinian Diagnosis

In this paper I offer a diagnosis of the inconsistency manifested in Frege’s writings between the idea that truth-functionally equivalent but structurally distinct sentences express the same thought and the principle of sense composition. Frege was under the influence of an analogy between how the sense of logical and that of non-logical components contributes to the sentence of which it is part. I show that in order to avoid the inconsistency, he should have restricted the principle of sense composition in the following way: the sense of any part of a sentence other than a sentential operator is part of the sense of that sentence. My diagnosis is based on one of the fundamental ideas of Wittgenstein’s Tractatus, i.e. that the sense of a logical component of a sentence does not characterize the sense of that sentence.