Full Idea
The usual general term, whether a common noun or a verb or an adjective, owes its generality to some resemblance among the things referred to.
Gist of Idea
General terms depend on similarities among things
Source
Willard Quine (Natural Kinds [1969], p.116)
Book Reference
Quine,Willard: 'Ontological Relativity and Other Essays' [Columbia 1969], p.116
A Reaction
Quine has a nice analysis of the basic role of similarity in a huge amount of supposedly strict scientific thought.