Full Idea
If similarity has no degrees there is no containing of kinds within broader kinds. If colored things are a kind, they are similar, but red things are too narrow for a kind. If red things are a kind, colored things are not similar, and it's too broad.
Gist of Idea
If similarity has no degrees, kinds cannot be contained within one another
Source
Willard Quine (Natural Kinds [1969], p.118)
Book Reference
Quine,Willard: 'Ontological Relativity and Other Essays' [Columbia 1969], p.118
A Reaction
[compressed] I'm on Quine's side with this. We glibly talk of 'kinds', but the criteria for sorting things into kinds seems to be a mess. Quine goes on to offer a better account than the (diadic, yes-no) one rejected here.
Related Idea
Idea 16936 Comparative similarity allows the kind 'colored' to contain the kind 'red' [Quine]