On the Equivalence Conjecture for Proof-Theoretic Harmony

The requirement of proof-theoretic harmony has played a pivotal role in a number of debates in the philosophy of logic. Different authors have attempted to precisify the notion in different ways. Among these, three proposals have been prominent in the literature: Harmony-as-conservative extension, Harmony-as-levelling procedure and Tennant’s Harmony-as-deductive equilibrium. In this paper I propose to clarify the logical relationships between these accounts. In particular, I demonstrate that what I call the equivalence conjecture—that these three notions essentially come to the same thing—is erroneous.