Full Idea
It seems that when subjects are not categorising by similarity, they are relying on what properties objects can and cannot have - that is, on some modal knowledge.
Gist of Idea
If categorisation is not by similarity, it seems to rely on what properties things might have
Source
Edouard Machery (Doing Without Concepts [2009], 6.5.1)
Book Reference
Machery,Edouard: 'Doing Without Concepts' [OUP 2009], p.185
A Reaction
I would call this essentialist categorisation, based on the inner causal powers which generate the modal profile of the thing. We categorise bullets and nails very differently, because of their modal profiles.