Full Idea
Abstracting takes place in two ways: by composition and division, understanding something to be not in another or to be separated from it; and by a simple and unconditioned consideration, understanding one thing while not considering the other at all.
Gist of Idea
Abstracting either treats something as separate, or thinks of it separately
Source
Thomas Aquinas (Summa Theologicae [1265], Q85 1 Ad 1)
Book Reference
Aquinas,Thomas: 'Treatise on Human Nature (Summa 1a 75-89)', ed/tr. Pasnau,Robert [Hackett 2002], p.159
A Reaction
The second way is by 'ignoring', which he says cannot contain error. The first seems to be considering some mode of a thing to be actually separate from the thing, which could clearly be erroneous. Ignoring makes to commitment to a unity.