Is it transitive? Does "continent cannot be island" imply "island cannot be continent"? I don't even know the other word to describe "island cannot be a single

Is it transitive? Does "continent cannot be island" imply "island cannot be continent"? I don't even know the other word to describe "island cannot be a single continent, but it CAN be multiple continents". I guess that gets into the question: "Can a river count as the border of an island?". What happens if the river dries up sometimes? It seems people have already decided there are 7 continents, and will adjust the definition until it gives the result.