Full Idea
Mediaeval logicians had a principle, 'Eadem est scientia oppositorum': in order to attain a clear conception of what it is for A to be the case, one needs to attain a conception of what it is for A not to be the case.
Gist of Idea
Medieval logicians said understanding A also involved understanding not-A
Source
Ian Rumfitt (The Boundary Stones of Thought [2015], 7.2)
Book Reference
Rumfitt,Ian: 'The Boundary Stones of Thought' [OUP 2015], p.196
A Reaction
Presumably 'understanding' has to be a fairly comprehensive grasp of the matter, so understanding the negation sounds like a reasonable requirement for the real thing.