"inter-" means between, "intra-" means within, "extra-" means outside. "intra-" and "inter" aren't quite synonyms but they definitely aren't opposites of eachother.
"inter-" means between, "intra-" means within, "extra-" means outside. "intra-" and "inter" aren't quite synonyms but they definitely aren't opposites of eachother.
Inter- implies relationships between entities, intra- implies relationships within entities.
In any single sentence context they cannot refer to the same relationships, and that which they are not is precisely the domain of the other word: they are true antonyms.
> and that which they are not is precisely the domain of the other word
External relationships are a thing, which are in neither of those domains.
An intramural activity is not the same thing as an intermural activity, but the overwhelming majority of activities which are not intramural activities are not intermural activities either, most are extramural activities.
> External relationships are a thing, which are in neither of those domains.
They are a subset of inter-, pretty obviously.