We now reach the climax of the whole process, effort, the actual sending forth through the deliberately chosen channel of the ideal desired and decided on. To it all the rest is merely preliminary, and in it the final move is made which commits us to the deed. About it, therefore, we may well desire the completest information. To tell the truth, I have none to give, and nobody else has. The nature of the operation is substantially unknown. Though something which we have been performing all day long, we and all our ancestors, no one of us has succeeded in getting a good sight of what actually takes place. Our purposes are prepared as I have described, and then those purposes--something altogether mental--change on a sudden to material motions. How is the transmutation accomplished? How do we pass from a mental picture to a set of motions in the physical world? What is the bridge connecting the two? The bridge is always down when we direct our gaze upon it, though firm when any act would cross. Nor can we trace our passage any more easily in the opposite direction. When my eyes are turned on my watch, for example, the vibrations of light striking its face are reflected on the pupil of my eye. There the little motions, previously existing only in the surrounding ether, are communicated to my optic nerve. This vibrates too, and by its motion excites the matter of my brain, and then--well, I have a sensation of the white face of my watch. But what was contained in that _then_ is precisely what we do not understand. Incoming motions may be transmuted into thought; or, as in effort, outgoing thought may be transmuted into motion. But alike in both cases, on the nature of that transmutation, the very thing we most desire to know, we get no light. In regard to this crucial point no one, materialist or idealist, can offer a suggestion. We may of course, in fault of explanation, restate the facts in clumsy circumlocution. Calling thought a kind of motion, we may say that in action it propagates itself from the mind through the brain into the outer world; while in the apprehension of an idea motions of the outer world pass into the brain, and there set up those motions which we know as thought. But after such explanations the mystery remains exactly where it was before. How does a "mental motion" come out of a bodily motion, or a bodily from a mental? It is wiser to acknowledge a mystery and to mark the spot where it occurs.