Full Idea
We are on stronger grounds in asserting the general existence of sums when considering pluralities and masses than when considering individuals.
Gist of Idea
Sums are more plausible for pluralities and masses than they are for individuals
Source
Peter Simons (Parts [1987], 5.2)
Book Reference
Simons,Peter: 'Parts: a Study in Ontology' [OUP 1987], p.186
A Reaction
I was thinking that the modern emphasis on referring to plurals was precisely to resist the idea that we must 'sum' them into one thing. If so, we wouldn't want to then sum several plurals. If a mass isn't a sum, how can we sum some masses?