No one can doubt that function logically precedes structure; or if any one does doubt this, he need only observe his own experience and see how in every new acquisition of knowledge or of power there come, first, the thought, the idea, then the effort, next the habit, and finally the modification of cerebral mechanism, in which the effort and the habit become represented in relatively permanent and static form. In fact, the crux of the whole discussion between science and metaphysics turns on, or harks back to the discussion between function and structure; and it is the latter, in the sense in which I mean the word, that has had of late a too large share of our attention. The enterprise on which we are all of us embarked,--whether we define it as an investigation, pure and simple, into human nature and human motives, or as a therapeutic attempt to relieve invalids of their symptoms,--is a larger one than it is commonly conceived of as being. Each physician and each investigator has, indeed, the right to say that for practical reasons he prefers to confine his attention to some single portion of one or the other of these tasks, be it never so small. But each one should regard himself as virtually under an obligation to recognize the respects in which this chosen task is incomplete. Every physicist is aware that there is some form of energy underlying, or rather expressing itself in, light and heat and gravitation. Physicists do not study this form of energy, not because they do not wish to but simply because they cannot do so by the only methods that they are allowed to use. But, as a reaction of defense, they sometimes assert that no one else can do so either, that this underlying energy cannot be explained. To say this is, however, in my judgment, to misappreciate what an explanation is.