Open Problems in Proof-Theoretic Semantics
Abstract I present three open problems the discussion and solution of which I consider relevant for the further development of proof-theoretic semantics: (1) The nature of hypotheses and the problem of the appropriate format of proofs, (2) the problem of a satisfactory notion of proof-theoretic harmony, and (3) the problem of extending methods of proof-theoretic semantics beyond logic.
Proof-theoretic semantics is the attempt to give semantical definitions in terms of proofs. Its main rival is truth-theoretic semantics, or, more generally, semantics that treats the denotational function of syntactic entities as primary. However, since the distinction between truth-theoretic and proof-theoretic approaches is not as clear cut as it appears at first glance, particularly if 'truth-theoretic' is understood in its modeltheoretic setting (see Hodges , and Došen ). it may be preferable to redirect attention from the negative characterisation of proof-theoretic semantics to its positive delineation as the explication of meaning through proofs. Thus, we leave aside the question of whether alternative approaches can or do in fact deal with the phenomena that proof-theoretic semantics tries to explain. In proof-theoretic semantics, proofs are not understood simply as formal derivations, but as entities expressing arguments by means of which we can acquire knowledge. In this sense, proof-theoretic semantics is closely connected and strongly overlaps with what Prawitz has called general proof theory.
The task of this paper is not to provide a philosophical discussion of the value and purpose of proof-theoretic semantics. For that the reader may consult SchroederHeister [56, 61] and Wansing . The discussion that follows presupposes some acquaintance with basic issues of proof-theoretic semantics. Three problems are addressed, which I believe are crucial for the further development of the prooftheoretic approach. This selection is certainly personal, and many other problems might be added. However, it is my view that grappling with these three problems opens up further avenues of enquiry that are needed if proof-theoretic semantics is to mature as a discipline.
The first problem is the understanding of hypotheses and the format of proofs. It is deeply philosophical and deals with the fundamental concepts of reasoning, but has important technical implications when it comes to formalizing the notion of proof. The second problem is the proper understanding of proof-theoretic harmony. This is one of the key concepts within proof-theoretic semantics. Here we claim that an intensional notion of harmony should be developed. The third problem is the need to widen our perspective from logical to extra-logical issues. This problem proceeds from the insight that the traditional preoccupation of proof-theoretic semantics with logical constants is far too limited.
I work within a conventional proof-theoretic framework where natural deduction and sequent calculus are the fundamental formal models of reasoning. Using categorial logic, which can be viewed as abstract proof theory, many new perspectives on these three problems would become possible. This task lies beyond the scope of what can be achieved here. Nevertheless, I should mention that the proper recognition of categorial logic within proof-theoretic semantics is still a desideratum. For the topic of categorial proof theory the reader is referred to Došen's work, in particular to his programmatic statement of 1995 , his contribution to this volume  and the detailed expositions in two monographs [7, 12].