Full Idea
Crowds seem to be the bearers of properties, and if they are things at all, then there is no place for them other than in the category of Substance.
Gist of Idea
If crowds are things at all, they seem to be Substances, since they bear properties
Source
Robert Pasnau (Metaphysical Themes 1274-1671 [2011], 26.1)
Book Reference
Pasnau,Robert: 'Metaphysical Themes 1274-1671' [OUP 2011], p.609
A Reaction
It is tempting to say, based on Aristotle, that a substance is whatever 1) bears properties, and 2) endures in spite of change, but a crowd is a nice problem case, because it looks too disunited to be a 'substance'.