Full Idea
The agent mind must itself turn to images, and produce by its own power in the receptive mind a representation as to species of whatever the images represent as individual.
Gist of Idea
The mind must produce by its own power an image of the individual species
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.134
A Reaction
Unlike much of this section, this sentence supports Geach's claim that Aquinas agrees with him - that the mind creates its concepts, rather than 'abstracting' them from experience.