Full Idea
Mass terms and plural terms differ principally in the indifference of mass terms to matters of division. A mass term can be used irrespective of how, indeed whether, the denotatum comes parcelled in units.
Gist of Idea
Mass terms (unlike plurals) are used with indifference to whether they can exist in units
Source
Peter Simons (Parts [1987], 6.4)
Book Reference
Simons,Peter: 'Parts: a Study in Ontology' [OUP 1987], p.234
A Reaction
It seems more to the point to say that mass terms (stuff) don't need units to exist, and you can disperse the units (the cups of water) without affecting the identity of the stuff. You can't pulverise a pile of stones and retain the stones.