Another way to phrase it is to note that the question is answered by the question of what happens when the wand is dropped and the goblin picks up the wand. Can it cast the spell? Then it belongs to the wand. If not then it belongs to the wizard.
In this case it just mean you have to match your game semantics to the decision of where to put the ability. That doesn't mean it would be a good idea to perform fighting by simple method calls on the objects though. It is very likely one would want to extract the abilities granted by skills and object to a separate entity that change seldom enough.
That doesn't mean I see what problems a schema solves here. That mostly seems like unecessary indirection..
Also note I don't try to refute the problem in general, just this specific example. Maths have lots of examples that doesn't have the semantic baggage that makes the question solvable.
