⊃E is Admissible in “true” relevant arithmetic

Abstract

The system R## of “true” relevant arithmetic is got by adding the ω-rule “Infer ∀xAx from A0, A1, A2, ....” to the system R# of “relevant Peano arithmetic”. The rule ⊃E (or “gamma”) is admissible for R##. This contrasts with the counterexample to ⊃E for R# (Friedman & Meyer, “Whither Relevant Arithmetic”). There is a Way Up part of the proof, which selects an arbitrary non-theorem C of R## and which builds by generalizing Henkin and Belnap arguments a prime theory T which still lacks C. (The key to the Way Up is a Witness Protection Program, using the ω-rule.) But T may be TOO BIG, whence there is a Way Down argument that produces a better theory TR, such that R## ⫅ TR ⫅ T. (The key to the Way Down is a Metavaluation, on which membership in T is combined with ordinary truth-functional conditions to determine TR.) The result is a theory that is Just Right, whence it never happens that A ⊃ C and A are theorems of R## but C is a non-theorem.