Full Idea
Hume needs a notion of resemblance where some things resemble a given thing more than other things do, and some may resemble exactly, and some hardly at all.
Gist of Idea
Hume needs a notion which includes degrees of resemblance
Source
comment on David Hume (Treatise of Human Nature, + Appendix [1740]) by Sydney Shoemaker - Causality and Properties §02
Book Reference
Shoemaker,Sydney: 'Identity, Cause and Mind' [OUP 2003], p.209
A Reaction
An astute and simple point. Once you admit degrees of resemblance, of course, then resemblance probably ceases to be a primitive concept in your system, and Hume would be well stuck.