Fr. Coplestone in his debate with A.J. Ayer seems to suggest that metaphysics “must, in my opinion, refer to a reality beyond or behind phenomena.” Thus, phenomena are the proper items of empirical science, whereas questions concerning what explains phenomena are metaphysical questions. He also says, in his more famous debate with Bertrand Russell, that:
Take the proposition “if there is a contingent being then there is a Necessary Being.” I consider that that proposition hypothetically expressed is a necessary proposition. If you are going to call every necessary proposition an analytic proposition, then — in order to avoid a dispute in terminology — I would agree to call it analytic, though I don’t consider it a tautological proposition. But the proposition is a necessary proposition only on the supposition that there is a contingent being. That there is a contingent being actually existing has to be discovered by experience, and the proposition that there is a contingent being is certainly not an analytic proposition, though once you know, I should maintain, that there is a contingent being, it follows of necessity that there is a Necessary Being.
~ Fr. Coplestone
However, it seems to me that Alvin Plantinga suggests that our belief in the external world is a properly basic belief (he also suggests that belief in God is, or at least can be, a properly basic belief). I wonder, therefore, if we might legitimately say that our belief in the intelligibility of metaphysics is in fact properly basic. In other words, belief in metaphysics in general (not to be confused with the belief in any metaphysical system in particular) is held in a properly basic way. This could be by extension of our empirical knowledge that there are phenomena, in combination with our rational intuition which obviates the inference to things which lie behind phenomena as an analytic inference. It is difficult to see what a defeater for this view would look like for any person to whom metaphysical questions and postulates are intelligible. This is hardly an apologetic to any logical empiricist, but it does express, to my mind, what the attitude of the metaphysician ought to be with respect to the legitimacy of metaphysics.
If belief in the external world, along with the relation of phenomena to noumena, to use Kantian vocabulary, are properly basic beliefs, then it seems as though metaphysics in general is necessarily also entertained in a properly basic way. Metaphysics consists only in the suggestion that behind phenomena lie noumena which exist (it is in the business of applying the existential predicate), and metaphysical systems simply propose different models of what aggregate of noumena exist.