This paper introduces a natural deduction calculus for intuitionistic logic of belief \ which is easily turned into a modal \-calculus giving a computational semantics for deductions in \. By using that interpretation, it is also proved that \ has good proof-theoretic properties. The correspondence between deductions and typed terms is then extended to a categorical semantics for identity of proofs in \ showing the general structure of such a modality for belief in an intuitionistic framework.

Structuralist foundations of mathematics aim for an ‘invariant’ conception of mathematics. But what should be their basic objects? Two leading answers emerge: higher groupoids or higher categories. I argue in favor of the former over the latter. First, I explain why to choose between them we need to ask the question of what is the correct ‘categorified’ version of a set. Second, I argue in favor of groupoids over categories as ‘categorified’ sets by introducing a pre-formal understanding of groupoids as (...) abstract shapes. This conclusion lends further support to the perspective taken by the Univalent Foundations of mathematics. (shrink)