Full Idea
As yet we have no idea of any third sort of thing that is neither individual nor class nor mixture of the two.
Gist of Idea
We have no idea of a third sort of thing, that isn't an individual, a class, or their mixture
Source
David Lewis (Parts of Classes [1991], 1.2)
Book Reference
Lewis,David: 'Parts of Classes' [Blackwell 1991], p.8
A Reaction
You can see that Lewis was a pupil of Quine. I quote this to show how little impression 'stuff' makes on the modern radar. His defence is that stuff may not be a 'thing', but then he seems to think that 'things' exhaust reality (top p.8 and 9).