Geach in his Mental Acts impugns the Lockean theory of concept formation which he calls abstractionism. To clarify his position vis a vis the Thomist view of abstraction he added an appendix:
HISTORICAL NOTE ON §11: AQUINAS AND ABSTRACTIONISM*
SOME PEOPLE may have been a good deal surprised at my using arguments adapted from Aquinas to refute abstractionism. Aquinas is very often regarded as an abstractionist, and many of his professed followers are abstractionists; and of course he does use the term "abstractio" for the process of forming concepts. All the same it can be decisively shown that in his maturest work, the Summa Theologica, his views are opposed to what I have called abstractionism.
In accepting the comparison whereby the intellectus agens, the mind's concept-forming power, is likened to a light that enables the mind's eye to see the intelligible features of things, as the bodily eye sees colours, Aquinas is careful to add that this comparison goes on all fours only if we suppose that colours are generated by kindling the light—that the light is not just revealing colours that already existed in the dark (la q. 79 art. 3 ad 2 um). Furthermore he says that when we frame a judgment expressed in words, our use of concepts is to be compared, not to seeing something, but rather to forming a visual image of something we are not now seeing, or even never have seen (la q. 85 art. 2 ad 8 um). So he expresses anti-abstractionist views both on the formation and on the exercise of concepts.
Again, an abstractionist, as we say, cannot allow that we possess 'proper' concepts of the various kinds of material substance in our environment, and it is arguable that he ought to reject the term "substance" as nonsensical. But it is a main thesis in Aquinas's theory of knowledge that what our understanding grasps primarily and most readily is the specific nature (quod quid est) of material substances, in spite of his also holding that the senses are in no way cognizant of this nature. In fact, he greatly exaggerates the ease and certainty of this knowledge; as when he says that the term "stone" signifies the nature of stone as it is in itself, since it expresses the definition of stone, by which we know what stone is (la q. 13 art. 8 ad 2 um). His soi-disant followers who adopt abstractionism have been obliged to reject his epistemology on this cardinal point; how they can think that this departure is not fundamental I have never understood.
I suppose it is the result of the empiricist tendency to believe that if we have got something e.g. a concept, it must be the result of our working on something in order to extract it.
Ed.Feser has a post on concept formation and its implications for materialist theories of the mind. sphexy