 Finally, a brief word on the models of SIA. These are the so-called smooth toposes, categories (see entry on  category theory)  of a certain kind in which all the usual mathematical operations can be performed but whose internal logic is intuitionistic and in which every map between spaces is smooth, that is, differentiable without limit. It is this “universal smoothness” that makes the presence of infinitesimal objects such as \(\Delta\) possible. The construction of smooth toposes (Moerdijk &amp; Reyes 1991) guarantees the consistency of SIA with intuitionistic logic. This is so despite the evident fact that SIA is not consistent with classical logic.