As for functors, some of them are simpler than others: extension can be attributed to them in a simple way. In case of a so-called "extensional" functor we can in a sense abstract from the "material" part of its inputs and output, and regard the functor as a function turning directly the "extension of" its input(s) into the extension of its output. Of course, it is assumed that we can do so at all: the extension of input expression(s) determines the extension of the resulting expression. Functors for which this assumption does not hold are called "intensional".