Full Idea
To say that a class is natural is to say that when some of its members are shown to people they pick out others without hesitation and in agreement.
Gist of Idea
A class is natural when everybody can spot further members of it
Source
Anthony Quinton (The Nature of Things [1973], 9 'Nat')
Book Reference
Quinton,Anthony: 'The Nature of Things' [RKP 1973], p.264
A Reaction
He concedes a number of problems with his view, but I admire his attempt to at least begin to distinguish the natural (real!) classes from the ersatz ones. A mention of causal powers would greatly improve his story.