Full Idea
If we think what defines a stone, man or horse, without thinking of any individual peculiarities it may have, this is precisely what we do when we abstract the general nature of what we understand from any particular way in which we imagine it.
Gist of Idea
We understand the general nature of things by ignoring individual peculiarities
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
This may not be simple abstraction from sense experience, since there would obviously be a threatened circularity in the process. Do you need to know the essential definition first, in order to discard the individual peculiarities?