Full Idea
To cognize that which is in individual matter, not as it is in such matter, is to abstract the form from the individual matter that the phantasms represents. Thus we come to a degree of cognition of immaterial things.
Clarification
'Phantasms' are appearances
Gist of Idea
We abstract forms from appearances, and acquire knowledge of immaterial things
Source
Thomas Aquinas (Summa Theologicae [1265], Q85 1 Reply)
Book Reference
Aquinas,Thomas: 'Treatise on Human Nature (Summa 1a 75-89)', ed/tr. Pasnau,Robert [Hackett 2002], p.158
A Reaction
This offers abstraction as a kind of inference to best explanation which takes us beyond immediate empirical experience to what is behind it. Aquinas thinks the concepts and explanation are spiritual, but they may be generalities and essences.