Abstract

A mathematical theory T is categorical if, and only if, any two models of T are isomorphic. If T is categorical, it can be shown to be semantically complete: for every sentence ϕ in the language of T, either ϕ follows semantically from T or ¬ϕ does. For this reason some authors maintain that categoricity theorems are philosophically significant: they support the realist thesis that mathematical statements have determinate truth-values. Second-order arithmetic (PA2) is a case in hand: it can be shown to be categorical and semantically complete. The status of second-order logic is a controversial issue, however. Worries about the purported set-theoretic nature and ontological commitments of second-order logic have been influential in the debate.