One argument, among others, from his al-Asfar al-Arba’a, vol.VIII, sec.V, chapter 4 reads:

Another [argument]: If the imaginative forms were imprinted in [a material organ e.g., the brain], as is the common [view] among the [followers of Galen], then either [a] each form will have a determined place which is not the place of [another] form [or [b] all those forms will be imprinted in one substratum]. [Option [a]], however, is absurd for one man can memorize volumes [of works], witness multiple climes, cities and their wonders and the forms of those things remain in his memory and imagination. It is known by self-evident knowledge that the brain cannot fulfil that [function]. [If option [b], then] the imagination would be like the tablet [on] which the sketches of some over the other[s] are inscribed without [them i.e., the imaginal forms] being [clearly] distinguished from each other. But the imagination is not like that. It witnesses them as distinct from each other without any confusion. So we learn that the [imaginative] forms are not imprinted [in a material organ].