Abstract

This chapter concerns that harmony is a particular relationship between the introduction rule and the elimination rule for a given connective. The Harmony Thesis says that a connective is defective unless its associated introduction and elimination rules are in harmony. It also says that a connective is defective if the logical principles which regulate its use go beyond a pair of harmonious introduction and elimination rules. The chapter scrutinizes the most influential arguments which have been put forward for the Harmony Thesis. It discusses Dummett‐Prawitz's argument for Harmony, which revealed the huge difficulties that confront the project of trying to explicate the notions of consequence and validity directly in terms of the rules which, for the Inferential Role Semantics (IRS) theorist, constitute the meanings of the connectives. The chapter concludes by discussing briefly how the failure of the Harmony Thesis affects the prospects for IRS.