Increasing sentences in Simple Type Theory

We introduce the notion of pseudo-increasing sentence, and prove that all such sentences are decidable by a weak subtheory of Simple Type Theory with infinitely many zero-type elements. We then present the consequences of this result to Quine's theory of “New Foundations” (NF). In particular, we prove the decidability of certain universal-existential sentences, and establish the consistency of a subtheory of NF.