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.