In good Fregean orthodoxy, a concept or a relation is scientifically acceptable only if

it has sharp boundaries, in other words, if it can be decided, for any object name filling

the empty place of the conceptual word, whether the resulting proposition is true or

false. It is well known that this requirement of sharp boundaries is the consequence

of a twofold thesis: first, a concept does not subsist outside a propositional context,

and second, a ‘scientific’ proposition is true or false, any third possibility being