The difference between idealism and materialism amounts to topology. It is in how the objects or furniture of experience is internally ordered, or a matter of the possible pathways that can be taken between distinct positions. If the thing is equivalent to the idea of the thing then this ordering goes along lines of similarity or archetype, otherwise it is along purely historical and literally contingent lines. The first means that something like magic must be effective, at least if you are able to go deep enough, otherwise certainly not etc. But, of course if materialism is itself and idea, then it would only amount to the claim that all effective lines of relation must be 'factored' through the idea of an external world, which idea itself has its own ideal relations to other cosmic paradigms. Simple arguments won't work, but there is no knock out.