Full Idea
From matter and form comes one thing per se. This is not so for subject and accident. Matter and form are instrinsic causes of a composite being, but whiteness and a human being are not. Humans can exist without whiteness, so it is one thing per accidens.
Gist of Idea
Matter and form give true unity; subject and accident is just unity 'per accidens'
Source
John Duns Scotus (Oxford Commentary on Sentences [1301], II.12.1.14), quoted by Robert Pasnau - Metaphysical Themes 1274-1671
Book Reference
Pasnau,Robert: 'Metaphysical Themes 1274-1671' [OUP 2011], p.101
A Reaction
This isn't much of a theory, but at least it is focusing on an interesting question, and the distinction between genuinely unified, and unified by chance. Compare a loving couple with siblings who hate each other.