Full Idea
I agree entirely with Dummett that the right way to answer the question 'what is logic?' is to consider transitions between sentences.
Gist of Idea
Logic is based on transitions between sentences
Source
Dag Prawitz (Gentzen's Analysis of First-Order Proofs [1974], §04)
Book Reference
'A Philosophical Companion to First-Order Logic', ed/tr. Hughes,R.I.G. [Hackett 1993], p.230
A Reaction
I always protest at this point that reliance on sentences is speciesism against animals, who are thereby debarred from reasoning. See the wonderful Idea 1875 of Chrysippus. Hacking's basic suggestion seems right. Transition between thoughts.
Related Idea
Idea 1875 Dogs show reason in decisions made by elimination [Chrysippus, by Sext.Empiricus]