In proceeding to consider the opposite theories of Spiritualism and Materialism, it is before all else desirable to be perfectly clear upon the point of theory whereby they are essentially distinguished. This point is that which is raised by the question whether mind is the cause or the effect of motion. Both theories are dualistic, and therefore agree in holding that there is causation as between mind and motion: they differ only in their teaching as to the direction in which the causation proceeds. Of course, out of this fundamental difference there arise many secondary differences. The most important of these secondary differences has reference to the nature of the eternal or self-existing substance. Both theories agree that there is such a substance; but on the question whether this substance be mental or material, the two theories give contradictory answers, and logically so. For, if mind as we directly know it (namely, in ourselves) is taken to be a cause of motion, within our experience mind is accredited with priority; and hence the inference that elsewhere, or universally, mind is prior to motion. Furthermore, as motion cannot take place without something which moves, this something is likewise supposed to have been the result of mind: hence the doctrine of the creation by mind both of matter and of energy. On the other hand, the theory of materialism, by refusing to assign priority to mind as known directly in ourselves, naturally concludes that mind is elsewhere, or universally, the result of matter in motion—in other words, that matter in motion is the eternal or self-existing substance, and, as such, the cause of mind wherever mind occurs.

I may observe, in passing, that although this cosmical deduction from the theory of materialism is, as I have said, natural, it is not (as is the case with the corresponding deduction from the theory of spiritualism) inevitable. For it is logically possible that even though all known minds be the results of matter in motion, matter in motion may nevertheless itself be the result of an unknown mind. This, indeed, is the position virtually adopted by Locke in his celebrated controversy with the Bishop of Worcester. Having been taken to task by this divine for the materialistic tendency of his writings, Locke defends himself by denying the necessary character of the deduction which we are now considering. For example, he insists, 'I see no contradiction in it that the first eternal thinking being should, if he pleased, give to certain systems of created senseless matter, put together as he thinks fit, some degrees of sense, perception, and thought: though, as I think, I have proved (lib. IV, ch. 10 and 14 &c.), it is no less than a contradiction to suppose matter (which is evidently in its own nature void of sense and thought) should be that eternal first thinking being.' Under this view, it will be observed, mind is supposed to have the ultimate priority, and thus to have been the original or creating cause of matter in motion, which, in turn, becomes the cause (or, at least, the conditional condition) of mind of a lower order. This view, however, need not detain us, inasmuch as it can only be held by those who, on grounds independent of philosophical thinking, already believe in mind as the First Cause or Eternal Being: this belief granted, there is, of course, an end of any question as between Spiritualism and Materialism. I have, therefore, only mentioned this possible phase of spiritualistic theory, in order to show that the theory of Materialism as applied to a human being does not necessarily involve an extension of that theory to the cosmos. But I hold this distinction as of no practical value: it merely indicates a logical possibility which no one would be likely to entertain except on grounds independent of those upon which the philosophical dispute between Spiritualism and Materialism must be confined.

Of more practical importance is the remark already made, namely, that the fundamental or diagnostic distinction between these two species of theory consists only in the views which they severally take on the question of causality. This remark is of practical importance, because in the debate between spiritualists and materialists it is often lost sight of: nay, in some cases, it is even expressly ignored. Obviously, when it is either intentionally or unintentionally disregarded, the debate ceases to be directed to the question under discussion, and may then wander aimlessly over the whole field of collateral speculation. Throughout the present essay, therefore, the discussion will be restricted to the only topic which we have to discuss—namely, whether mind is the cause of motion, motion the cause of mind, or neither the cause of the other.

The view to be first considered—namely, that mind is the cause of motion—obviously has one great advantage over the opposite view: it supposes the causality to proceed from that which is the source of our idea of causality (the mind); not from that into which this idea has been read by the mind. Hence, it is so far less difficult to imagine that mental changes are the cause of bodily changes than vice versa; for upon this hypothesis we are starting at least from the substance of immediate knowledge, and not from the reflection of that knowledge in what we call the external world.

On the other hand, the theory of Spiritualism labours under certain speculative difficulties which appear to me overwhelming. The most formidable of these difficulties arises from the inevitable collision of the theory with the scientific doctrine of the conservation of energy. Whether or not we adopt the view that all causation of a physical kind is ultimately an expression of the fact that matter and energy are indestructible[3], it is equally certain that this indestructibility is a necessary condition to the occurrence of causation as natural. Therefore, if the mind of man is capable of breaking in as an independent cause upon the otherwise uniform system of natural causation, the only way in which it could do so would be by either destroying or creating certain quanta of either matter or energy or both. But to suppose the mind capable of doing any of these things would be to suppose that the mind is a cause in some other sense than a physical or a natural cause; it would be to suppose that the mind is a super-natural cause, or, more plainly, that all mental activity, so far as it is an efficient cause of bodily movement, is of the nature of a miracle.

This conclusion, which appears to me unavoidably implicated in the spiritualistic hypothesis, is not merely improbable per se, but admits of being shown virtually impossible if we proceed to consider the consequences to which it necessarily leads. A sportsman, for example, pulls the trigger of a gun, thereby initiating a long train of physical causes, which we may take up at the point where the powder is discharged, the shot propelled, and the bird dropped. Here the man's volition is supposed to have broken in upon the otherwise continuous stream of physical causes—first by modifying the molecular movements of his brain, so as to produce the particular co-ordination of neuro-muscular movement required to take accurate aim and to fire at the right moment; next by converting a quantity of gunpowder into gas, propelling a quantity of lead through the air; and finally, by killing a bird. Now, without tracing the matter further than this, let us consider how enormous a change the will of the man has introduced, even by so trivial an exercise of its activity. No doubt the first change in the material world was exceedingly slight: the molecular movement in the cortex of his brain was probably not more than might be dynamically represented by some small fraction of a foot-pound. But so intricate is the nexus of physical causality throughout the whole domain of Nature, that the intervention of even so minute a disturbance ab extra is obviously bound to continue to assert an influence of ever-widening extent as well as of everlasting duration. The heat generated by the explosion of the powder, the changed disposition of the shot, the death of the bird—leading to innumerable physical changes as to stoppage of many mechanical processes previously going on in the bird's body, loss of animal heat, &c., and also to innumerable vital changes, leading to a stoppage of all the mechanical changes which the bird would have helped to condition had it lived to die some other death, to propagate its kind, and thus indirectly condition an incalculable number of future changes that would have been brought about by the ever increasing number of its descendants—these and an indefinite number of other physical changes must all be held to have followed as a direct consequence of the man's volition thus suddenly breaking in as an independent cause upon the otherwise uniform course of Nature. Now, I say that, apart from some system of pre-established harmony, it appears simply inconceivable that the order of Nature could be maintained at all, if it were thus liable to be interfered with at any moment in any number of points. And if the spiritualist takes refuge in the further hypothesis of a pre-established harmony between acts of human (not to add brute) volition and causes of a natural kind, we have only to observe that he thus lands himself in a speculative position which is practically identical with that occupied by the materialist. For the only difference between the two positions then is that the necessity which the materialist takes to be imposed on human volition by the system of natural causation, is now taken by the spiritualist to be equally imposed by a super-natural volition. The necessity which binds the human volition must be equally rigid in either case; and therefore it can make no practical difference whether the source of it be regarded as natural or super-natural, material or mental: so that a man be fated to will only in certain ways—and this with all the rigour which belongs to causation as physical—it is scarcely worth while to dispute whether the predestination is of God or of Nature. There can be no question, however, that in this matter the possibility which I have supposed to be suggested by the spiritualist is more far-fetched than that which obviously lies to the hand of the materialist; and, moreover, that it too plainly wears the appearance of a desperate device to save a hollow theory.

It remains to add that this great difficulty against the spiritualistic theory has been revealed in all its force only during the present generation. Since the days of fetishism, indeed, the difficulty has always been an increasing one—growing with the growth of the perception of uniformity on the one hand, and of mechanical as distinguished from volitional agency on the other. But it was not until the correlation of all the physical forces had been proved by actual experiment, and the scientific doctrine of the conservation of energy became as a consequence firmly established, that the difficulty in question assumed the importance of a logical barrier to the theory of mental changes acting as efficient causes of material changes.