Full Idea
Aquinas subscribes to the unitarian doctrine that a single substance has just a single substantial form, but authors like Scotus subscribe to a plurality of substantial forms.
Gist of Idea
Aquinas says a substance has one form; Scotists say it has many forms
Source
Robert Pasnau (Metaphysical Themes 1274-1671 [2011], 24.1)
Book Reference
Pasnau,Robert: 'Metaphysical Themes 1274-1671' [OUP 2011], p.550
A Reaction
The Scotists seem to think that qualities themselve can have forms. I take it that Aristotle would have agreed with Aquinas.