Full Idea
Abstacting A from B can mean denying A's connection with B, or simply thinking A without thinking B. Abstracting what in reality is connected generates falsehood if done the first way, but not if done the second.
Gist of Idea
Abstracting A from B generates truth, as long as the connection is not denied
Source
Thomas Aquinas (Summa Theologicae [1265], Ch.5 Q85.1)
Book Reference
Aquinas,Thomas: 'Summa Theologicae (Concise)', ed/tr. McDermott,Timothy [Christian Classics 1991], p.133
A Reaction
Despite Geach's denials, this seems to make Aquinas a classic abstractionist. He goes on to distinguish two sorts of abstraction, but he certainly thinks of abstraction from sense experience as a revelation about the nature of reality.