We come, then, to the question of the rationale of this correlation, and it is clear here that we are dealing with a phase of the problem of the ultimate nature of reality. For the question of the correlation now is how it is possible that our thoughts should affect things so that they move in response; how mind influences body or the reverse, how, when we will, the arm moves through space. And without going into details of discussion here, let us say at once, that whatever the situation may be for any science,--and it may be that some form of _dualism_ is a necessary presupposition of science,--for metaphysics it is clear that no dualism of substances or orders can be regarded as final. The life of metaphysics depends on finding the one for the many; the one that when found will also ground the many. If, then, the phenomenon of _mind and body_ presents the appearance of a correspondence of two different and, so far as can be determined, mutually exclusive agencies, the problem of metaphysics is the reduction of these agencies to one species. Here we come upon the issue between materialism and immaterialism. But inasmuch as the notion of metaphysics itself seems to exclude materialism, the vital alternative is that of immaterialism. Again, if psycho-physics presents as its basal category a _parallelism_ between two orders of phenomena, psychic and physical, it is the business of metaphysics to seek the explanation of this dualism in some more ultimate and unitary conception. Now, since the very notion of metaphysics again excludes the physical alternative from the category of finality, we are left with the psychic term as the one that, by virtue of the fact that it embodies a form of _conscious_ activity, promises to be most fruitful for metaphysics. From one point of view, then, we have reduced our world to immaterialism; from another, to some form or analogue of the psychic. Now it is not necessary here to carry the inquiry further in this direction. For what metaphysics is interested in, specially, is the fact that the world must be reduced to one kind of being and one type of agency. If this be done, it is clear that the dualism of _body and mind_ and the _parallel orders_ of psycho-physics cannot be regarded as final, but must take their places as phenomena that are relative and reducible to a more fundamental unity. The metaphysician will say that the arm moves through space in response to the will, and that everywhere the correlation between mechanical and teleological agency takes place because in the last analysis _there is only one type of agency_; an agency that finds its initiative in interest, thought, purpose, design, and thus works out its results in the fields of space and mechanical activities.