I.—“I believe,” says Professor Tyndall, “that all the great thinkers who have studied the subject are ready to admit the following hypothesis, that every conscious act, whether in the domain of the senses, of thought, or of emotion, corresponds to a certain definite molecular state of the brain; that this relation of the physical to consciousness invariably exists, so that, given the state of the brain, we may deduce from it the corresponding thought or feeling; or given the thought or feeling, we may deduce from it the state of the brain. But how to make this deduction ? At bottom it is not a case of logical deduction; it is, in fact, a case of empirical association. It may be replied that very many of the deductions of science have this empirical character; such is that by which it is affirmed that an electric current circulating in a given direction will make the magnetic needle deviate in a definite direction. But the two cases differ in this, that if the influence of the current upon the needle cannot be shown, it can at least be imagined, and that we have no doubt whatever that ultimately the problem will be mechanically resolved, while we cannot even picture to ourselves the passage from the physical state of the brain to the corresponding facts of feeling. Let it be admitted that a definite thought corresponds simultaneously to a definite molecular action in the brain. Well, we do not possess the intellectual organ, we have not even the rudiment apparently of the organ, which would enable us to pass by reasoning from one phenomenon to the other. They are produced together, but we know not why. If our intelligence and our senses were perfect enough, vigorous enough, enlightened enough to enable us to see and to feel the actual molecules of the brain; if we could trace all the movements, the groupings, the electric discharges, if they exist, of these molecules; if we knew perfectly the molecular states which correspond to this or that state of thought or of feeling, we should be still as far as ever from the solution of the problem: what is the connection between this physical state and the facts of consciousness? The abyss which exists between these two classes of phenomena will be always intellectually impassable. Let us admit that the feeling of love, for instance, corresponds to a spiral movement of the molecules of the brain to the right, and the feeling of hate to a spiral movement to the left, we should know that when we love, a movement is produced in one direction, and that when we hate, a movement is produced in another direction; but the why would remain still unanswered.”∗