 A particularly appealing idea is to pursue Hilbert’s program relative to a constructive point of view and determine which parts of classical mathematics are demonstrably consistent relative to that standpoint (see Rathjen 2009 for pursuing this with regard to Martin-Löf type theory). As one would suspect, there are differing “schools” of constructivism and different layers of constructivism. Several frameworks for developing mathematics from such a point of view have been proposed. Some we will refer to in this article (arguably the most important) are: