Full Idea
Either out of conviction or for reasons of algebraic neatness, classical extensional mereology asserts the existence of certain individuals, mereological sums, for whose existence in general we have no evidence outside the theory itself.
Gist of Idea
Classical mereology says there are 'sums', for whose existence there is no other evidence
Source
Peter Simons (Parts [1987], Intro)
Book Reference
Simons,Peter: 'Parts: a Study in Ontology' [OUP 1987], p.1
A Reaction
Observing that we have no evidence for sums 'outside the theory' is nice. It is a nice ontological test, with interesting implications for Quinean ontological commitment.