  Proof-theoretic validity is the dominating approach to proof-theoretic semantics. As a technical concept it was developed by Prawitz (1971; 1973; 1974), by turning a proof-theoretic validity notion based on ideas by Tait (1967) and originally used to prove strong normalization, into a semantical concept. Dummett provided much philosophical underpinning to this notion (see Dummett, 1991). The objects which are primarily valid are proofs as representations of arguments. In a secondary sense, single rules can be valid if they lead from valid proofs to valid proofs. In this sense, validity is a global rather than a local notion. It applies to arbitrary derivations over a given atomic system, which defines derivability for atoms. Calling a proof which uses an introduction rule in the last step canonical, it is based on the following three ideas: