Full Idea
A human being has no substantial form other than the intellective soul alone, and it contains the sensitive and nutritive souls, and all lower forms, and it alone brings about whatever it is that less perfect forms bring about in other things.
Gist of Idea
Humans only have a single substantial form, which contains the others and acts for them
Source
Thomas Aquinas (Summa Theologicae [1265], Ia Q76 4c), quoted by Robert Pasnau - Metaphysical Themes 1274-1671 25.1
Book Reference
Pasnau,Robert: 'Metaphysical Themes 1274-1671' [OUP 2011], p.575
A Reaction
He says brutes and plants also have a single soul. Pasnau says this is Aquinas's most distinctive doctrine, because other thinkers postulate a whole hierarchy of substantial forms.