Full Idea
Fuzzy logic is based upon fuzzy set-theory, in which the simple notion of membership of a set is replaced by a notion of membership to some degree.
Gist of Idea
Fuzzy logic is based on the notion that there can be membership of a set to some degree
Source
Thomas Mautner (Penguin Dictionary of Philosophy [1996], p.214)
Book Reference
Mautner,Thomas: 'Dictionary of Philosophy' [Penguin 1997], p.214
A Reaction
The idea that something could be to some degree a 'heap of sand' sounds plausible, but Williamson and Sorensen claim that the vagueness is all in us (i.e. it is epistemological), and not in the world. This will scupper fuzzy logic.