But materialists insist so much on the identity of brain-physiology with psychology, that even at the risk of tediousness we will multiply illustrations in order, if possible, to make this point still clearer. Suppose, then, we exposed the brain of a living man in a state of intense activity. Suppose, further, that our senses were absolutely perfect, so that we could see every change, of whatever sort, taking place in the brain-substance. What would we see? Obviously nothing but molecular changes, physical and chemical; for to the outside observer there is absolutely nothing else there to see. But the subject of this experiment sees nothing of all this. His experiences are of a different order, viz., consciousness, thought, emotions, etc. Viewed from the _outside_, there is--there can be--nothing but motions; viewed from the _inside_, nothing but thought, etc.--from the one side, only _physical_ phenomena; from the other side, only _psychical_ phenomena. Is it not plain that, from the very nature of the case, it must ever be so? Certain vibrations of brain molecules, certain oxidations with the formation of carbonic acid, water, and urea on the one side, and there appear on the other sensations, consciousness, thoughts, desires, volitions. There are, as it were, two sheets of blotting-paper pasted together. The one is the brain, the other the mind. Certain ink-scratches or blotches, _utterly meaningless_ on the one, soak through and appear on the other as _intelligible writing_, but how we know not, and can never hope to guess. But when the paste dissolves, _shall the writing remain_? We shall see. But some will object. There is nothing specially strange and unique in all this, for the same mystery underlies the essential nature of all kinds of force and matter, and therefore all phenomena. True enough, but with this difference. Physical and chemical forces and phenomena are indeed incomprehensible in their essential nature; but once accept their existence, and all their different forms are mutually convertible, construable in terms of each other and all in terms of motion. But it is impossible by any stretch of the imagination to thus construe mental forces and mental phenomena. It may, indeed, be impossible to conceive _how came_ the plane of material existence, but, standing on that plane, all phenomena fall into intelligible order. But there is another plane above this one, having no intelligible relation with it. We must climb up and stand on this before its phenomena fall into intelligible order. In a word, material forces and phenomena are, indeed, a mystery, but only of the _first order_. But mental and moral forces and phenomena are a mystery even from the standpoint of the other, and are therefore a mystery of the _second order_--a mystery within a mystery.