In no respect has the progress of physical science exercised a more profound influence upon philosophical thought than it has by proving an apparently quantitative relation between material changes and mental changes. It has always been known that there is qualitative relation. Even long before mankind suspected that the brain was in any way connected with thought, it was well understood that alcohol and other poisons exercised their sundry influences on the mind in virtue of influences which they exercised upon the body; and even the lowest savages must always have been aware that a blow on the head is followed by insensibility. But it was not until the rise of Physiology that this qualitative relation between corporeal changes and mental changes was gradually found to be a quantitative one—or that every particular change of mind had an exact and invariable counterpart in some particular change of body. It is needless for me to detail the successive steps in the long course of physiological discovery whereby this great fact has been established; it is enough to say that the fact is established to the satisfaction of every physiologist.

Now, when once the relation between material changes and mental changes has been thus recognized as quantitative—or, which is the same thing, when once the association has been recognized as both invariable and exact—there arises the question as to how this relation is to be explained. Formally considered—or considered as a matter of logical statement irrespective of the relative probabilities which they may present, either to the minds of different individuals or to the general intelligence of the race—it appears to me that the possible hypotheses are here seven in number.

So far as I can see, this list of possible answers to the question before us is exhaustive. I will next show why, in my opinion, the last four of them may be excluded in limine.

The suggestion of pre-established harmony (IV) merely postpones the question: it assumes a higher mind as adjusting correspondencies between known minds and animal bodies with respect to the activities of each; and, therefore, it either leaves untouched the ultimate question concerning the relation of mind (as such) to matter, or else it answers this question in terms of spiritualism (I).

The suggestion of chance (V) is effectually excluded by the doctrine of chances: even in any one individual mind, the association between mental changes and material changes is much too intimate, constant, and detailed to admit of any one reasonably supposing that it can be due only to chance.

The suggestion of pure idealism (VI) ultimately implies that the thinking Ego is itself the sole existence—a position which cannot, indeed, be turned by any assault of logic; but one which is nevertheless too obviously opposed to common sense to admit of any serious defence; its immunity from direct attack arises only from the gratuitous nature of its challenge to prove a negative (namely, that the thinking Ego is not the sole existence), and this a negative which is necessarily beyond the region of proof.

Lastly, the suggestion that the problem is necessarily insoluble (VII) does not deserve to be regarded as an hypothesis at all; for to suppose that the problem is necessarily insoluble is merely to exclude the supposition of there being any hypothesis available.

In view of these several considerations, it appears to me that, although in a formal sense we may say there are altogether seven possible answers to the question before us, in reality, or for the purposes of practical discussion, there are nowadays but three—namely those which head the above list, and which I will now proceed to consider.

I have named these three hypotheses in the order of their appearance during the history of philosophical thought. The earliest is the spiritualistic. As far back as we can trace the conceptions of primitive man, we meet with an unquestioning belief that it is his spirit which animates his body; and, starting from this belief as explanatory of the movements of his own body, he readily attributes movements elsewhere to analogous agencies—the theory of animism in Nature thus becoming the universal theory in all early stages of culture. It also appears to be the theory most natural to our own children during the early years of their dawning intelligence, and would doubtless continue through life in the case of every individual human being, were he not subsequently instructed in the reasons which have led to its rejection by many other members of his race. These reasons, as already observed, have been furnished in their entirety only within comparatively recent times; not until Physiology was able to prove how intimate is the association between cerebral processes and mental processes did it become possible for materialism to turn the tables upon spiritualism, by simply inverting the hypothesis. Lastly, although the theory of Monism (III) may be traced back at least as far as the pantheistic thought of Buddhism, it there had reference to theology as distinguished from psychology. And even as presented in the writings of Bruno, Spinoza, and other so-called monists prior to the present century, the hypothesis necessarily lacked completeness on account of the absence of knowledge afterwards supplied by physiology. For Monism, in the sense of this term as I shall use it, may be metaphorically regarded as the child of the two pre-existing theories, Spiritualism and Materialism. The birth of this child was necessarily impossible before both its parents had reached mature age. On the one hand it was necessary that the theory of Spiritualism should have outgrown its infancy as Animism, its childhood as Polytheism, before it entered upon its youth as Monotheism—or before it was able to supply material for the conception of Monism as a theory of cosmical extent. On the other hand, Materialism required to grow into the fullness of manhood, under the nursing influence of Science, before it was possible to engender this new-born offspring; for this offspring is new-born. The theory of Monism, as we are about to consider it, is a creature of our own generation; and it is only as such that I desire to call attention to the child. In order, however, to do this, I must follow the example of biographers in general, and begin by giving a brief sketch of both the parents.