The thesis that, in a system of natural deduction, the meaning of a logical constant is given by some or all of its introduction and elimination rules has been developed recently in the work of Dummett, Prawitz, Tennant, and others, by the addition of harmony constraints. Introduction and elimination rules for a logical constant must be in harmony. By deploying harmony constraints, these authors have arrived at logics no stronger than intuitionist propositional logic. Classical logic, they maintain, cannot be justified (...) from this proof-theoretic perspective. This paper argues that, while classical logic can be formulated so as to satisfy a number of harmony constraints, the meanings of the standard logical constants cannot all be given by their introduction and/or elimination rules; negation, in particular, comes under close scrutiny. (shrink)

We should address not the issue of the nature of truth, but rather that of the way we come by the very concept of truth. As Hilbert's discussion of quantifiers shows, the concept of truth is borne out of a more basic concept of justifiability. Our mastery of the most primitive aspects of the use of language to transmit information does not require even an implicit grasp of the concept of truth, but can be fully described in terms of the (...) antecedent notion of justifiability. (shrink)