Quotation by Stephen Toulmin

Definitions are like belts. The shorter they are, the more elastic they need to be. A short belt reveals nothing about its wearer: by stretching, it can be made to fit almost anybody. And a short definition, applied to a heterogeneous set of examples, has to be expanded and contracted, qualified and reinterpreted, before it will fit every case. Yet the hope of hitting on some definition which is at one and the same time satisfactory and brief dies hard: much can be learned by seeing how much elasticity is ultimately required of such portmanteau definitions.