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.