Full Idea
For many categories there is simply no definition to learn (such as Wittgenstein's example of a 'game').
Gist of Idea
Many categories don't seem to have a definition
Source
Edouard Machery (Doing Without Concepts [2009], 4.1.4)
Book Reference
Machery,Edouard: 'Doing Without Concepts' [OUP 2009], p.82