A note on restriction rules

Abstract

We consider a certain class of infinitary rules of inference, called here restriction rules, using of which allows us to deduce complete theories of given models. The first instance of such rules was the $\omega$-rule introduced by Hilbert, and generalizations of the $\omega$-rule were first considered by Henkin. Later on Barwise showed that within countable languages, for any countable model $\mathfrak M$, first-order logic expanded by the corresponding $\mathfrak M$-rule deduces from the diagram of $\mathfrak M$ all formulas that are true in all models that include $\mathfrak M$, in particular, it deduces the (relativized) complete theory of $\mathfrak M$. We show that, if the aim is only deducing the complete theory of a given model, these countability assumptions can be omitted. Moreover, similar facts hold for infinitary and higher-order logics, even if these logics are highly incomplete. Finally, we show that Barwise's theorem in its stronger form, for vocabularies of arbitrary cardinality, holds for infinitary logics $\mathscr L_{\kappa,\lambda}$ whenever $\kappa$ is supercompact. The note also contains brief historical remarks.