We present various results regarding the decidability of certain sets of sentences by Simple Type Theory. First, we introduce the notion of decreasing sentence, and prove that the set of decreasing sentences is undecidable by Simple Type Theory with infinitely many zero-type elements ; a result that follows directly from the fact that every sentence is equivalent to a decreasing sentence. We then establish two different positive decidability results for a weak subtheory of math formula. Namely, the decidability of math (...) formula and math formula. Finally, we present some consequences for the set of existential-universal sentences. All the above results have direct implications for Quine's theory of “New Foundations” and its weak subtheory math formula. (shrink)

Tangled Type Theory was introduced by Randall Holmes in [3] as a new way of approaching the consistency problem for$\mathrm {NF}$. Although the task of finding models for this theory is far from trivial (considering it is equiconsistent with$\mathrm {NF}$), ways of constructing models for certain fragments of it have been discovered. In this article, we present a simpler way of constructing models of predicative Tangled Type Theory and consequently of predicative$\mathrm {NF}$. In these new models of predicative$\mathrm {NF}$, the (...) universe is well-orderable and equinumerous to the set of singletons. (shrink)

We examine the conditions under which a model of Tangled Type Theory satisfies the same sentences as a model of (assuming we ignore type indices).