  “A definition of a concept (of a possible predicate) must […] unambiguously determine, as regards any object, whether or not it falls under the concept (whether or not the predicate is truly assertible of it). […] We may express this metaphorically as follows: the concept must have a sharp boundary. To a concept without sharp boundary there would correspond an area that had not a sharp boundary-line all around, but in places just vaguely faded away into the background. […] The law of the excluded middle is really just another form of the requirement that the concept should have a sharp boundary.” [Gottlob Frege, The Fundamental Laws of Arithmetic, Vol. II, §56 (1903: 159)]