Full Idea
The scholastic view is that Actuality is our only guide to possibility in the real order. One knows that it is possible to separate A and B if one knows that A and B have actually been separated or are separate.
Gist of Idea
Scholastics assess possibility by what has actually happened in reality
Source
report of Francisco Suárez (Disputationes metaphysicae [1597], Bk VII) by Stephen Boulter - Why Medieval Philosophy Matters 4
Book Reference
Boulter,Stephen: 'Why Medieval Philosophy Matters' [Bloomsbury 2019], p.102
A Reaction
It may be possible to separate A and B even though it has never happened, but it is hard to see how we could know that. (But if I put my pen down where it has never been before, I know I can pick it up again, even though this has not previously happened).