The medieval theories of logic generally hold, at least as represented in the brilliant work of Thomas Aquinas, that non-existent being can be said of existent being because non-existent being can be said to exist in some ontologically distinct manner from actually existent being; that is, it can be said to exist as a sort of image or concept in the mind. Thus, we can actually speak about non-existing things because they have some sort of existence in the mind.
But this seems, perhaps, to still not account for a gap in our reference. For the medieval logician would still likely offer that a proposition is true iff the form represented by the predicate actually inheres in the subject (aka, the inherence theory of predication). But how can our concept of such a non-existing thing properly be in relation to any existing thing? How can it be true that some non-existing thing inheres in an existing thing?
We might form a concept of the non-existing thing in which the actually non-existing thing takes some mitigated sense of existence but this does not change the fact that such a concept represents what is actually a non-existent, one that can be said to have a relation to an existent. This would needless to say require an interesting dissection of medieval relational ontology.
Have medieval philosophers offered clarity or insight into the way in which non-existents can inhere in reality? If they don't actually inhere in reality, then what accounts for the reference within our propositions that possess meaning despite being mere negations?
Disclaimer: It is very likely that I appear a fool in this question, for I am nearly illiterate in medieval logic. So if my question makes it seem that I am speaking poorly for medieval philosophers, such is very likely. It is not my intention to speak for any medieval philosophers at all though. Corrections and insight are welcome, especially as regards the initial presumptions I make even in the methodology by which I am asking.