So much for the physiologist in his capacity of pure physicist. As long as he remains behind the scenes in painful exploration of the details of the machinery—as long as he only observes the action of the players from behind the stage—so long will he miss the spirit of the performance, which is, nevertheless, caught easily by one who sees it from the front. May he not, then, for once in a way, be allowed to change his standpoint? True, he came not to see the representation of an imaginary world; he is in search of the actual; but surely it must help him to a comprehension of the dramatic apparatus itself, and of the manner in which it is worked, if he were to view its action from in front as well as from behind, or at least allow himself to hear what sober-minded spectators can tell him upon the subject. There can be no question as to the answer; and hence it comes that psychology is such an indispensable help to physiology, whose fault it only in small part is that she has hitherto made such little use of this assistance; for psychology has been late in beginning to till her fertile field with the plough of the inductive method, and it is only from ground so tilled that fruits can spring which can be of service to physiology. If, then, the student of nervous physiology takes his stand between the physicist and the psychologist, and if the first of these rightly makes the unbroken causative continuity of all material processes an axiom of his system of investigation, the prudent psychologist, on the other hand, will investigate the laws of conscious life according to the inductive method, and will hence, as much as the physicist, make the existence of fixed laws his initial assumption. If, again, the most superficial introspection teaches the physiologist that his conscious life is dependent upon the mechanical adjustments of his body, and that inversely his body is subjected with certain limitations to his will, then it only remains for him to make one assumption more, namely, _that this mutual interdependence between the spiritual and the material is itself also dependent on law_, and he has discovered the bond by which the science of matter and the science of consciousness are united into a single whole. Thus regarded, the phenomena of consciousness become functions of the material changes of organised substance, and inversely—though this is involved in the use of the word “function”—the material processes of brain substance become functions of the phenomena of consciousness. For when two variables are so dependent upon one another in the changes they undergo in accordance with fixed laws that a change in either involves simultaneous and corresponding change in the other, the one is called a function of the other. This, then, by no means implies that the two variables above-named—matter and consciousness—stand in the relation of cause and effect, antecedent and consequence, to one another. For on this subject we know nothing.