In at least two places (I believe there are more) in Frege: Philosophy of Language, Dummett says that Frege made a retrograde move. The first of these was in assimilating sentences to the class of complex terms. Sentences refer to objects, just like other terms, in Frege’s later philosohy; they refer to the truth and the false. This is opposed to their function in his early philosophy which is to have truth-conditions. What’s the difference here? Well, the prior is a matter of reference and the latter is a matter of meaning. That’s one big thing. The other is that on the complex term account, sentences lose their distinctive status. They are special in that they are what let us make moves in language games (what about the builders in the Philosophical Investigations?). They are things that we can take responsibility for, be committed to. Terms don’t satisfy these functions at all.

The other retrograde move Frege made was characterizing logic as studying truth rather than inference. I’m not sure if the tradition prior to Frege, of which Kant said was a completed science, focused on inference as in consequence. But, I am not familiar with the pre-Fregean tradition much at all. He blames this focus on truth as logic’s object of study for the theoretical eddy that was the analytic/synthetic discussion of the logical positivists. Focusing on truth led the positivists to distinguish two kinds of truths, namely the analytic and the synthetic, which could explain why logic was useful and had truths despite having no empirical content. I think this is an interesting observation on Dummett’s part. I’d like to check out the pre-Fregean tradition in logic and see how much it focused on inference or consequence over truth (massive display of ignorance here) and then reconsider his point.