Full Idea
Mental activity combines two activities which in the senses are distinct: exterior perception in which we are simply affected by what we sense, and interior imagination in which we create images of things that are not, and never have been present.
Gist of Idea
Mental activity combines what we sense with imagination of what is not present
Source
Thomas Aquinas (Summa Theologicae [1265], Ch.5 Q85.2)
Book Reference
Aquinas,Thomas: 'Summa Theologicae (Concise)', ed/tr. McDermott,Timothy [Christian Classics 1991], p.135
A Reaction
Geach cites this thought to show that he is anti-abstractionist, since mind creates images, and these can arise from things which have not been experienced. Any defence of abstractionism must allow an active power to imagination.