Full Idea
A nominalist definition of existence is 'having spatio-temporal location'.
Gist of Idea
A nominalist view says existence is having spatio-temporal location
Source
Robert C. Stalnaker (Mere Possibilities [2012], 1.1)
Book Reference
Stalnaker,Robert C.: 'Mere Possibilities' [Princeton 2012], p.4
A Reaction
This would evidently be physicalist as well as nominalist. Presumably it fits the 'mosaic' of reality Lewis refers to. I find this view sympathetic. A process of abstraction is required to get the rest of the stuff we talk about.