“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 the matter and the science of consciousness are united into a single whole.”