Full Idea
Crudely, the scientist posits only those entities without which she cannot account for observations, while the set theorist posits as many entities as she can, short of inconsistency.
Gist of Idea
Scientists posit as few entities as possible, but set theorist posit as many as possible
Source
Penelope Maddy (Naturalism in Mathematics [1997], II.5)
Book Reference
Maddy,Penelope: 'Naturalism in Mathematics' [OUP 2000], p.131