be able to glide in all directions over the bodies with which it is
in contact. Then with the consciousness of that mobility, the notion
of space of three dimensions--which is "_Raum_" or "room" to move with
perfect freedom--is at once given. But the notion that the tactile
surface itself moves, cannot be given by touch alone, which is
competent to testify only to the fact of change of place, not to its
cause. The idea of the motion of the tactile surface could not, in
fact, be attained, unless the idea of change of place were accompanied
by some state of consciousness, which does not exist when the tactile
surface is immoveable. This state of consciousness is what is termed
the muscular sense, and its existence is very easily demonstrable.

Suppose the back of my hand to rest upon a table, and a sovereign to
rest upon the upturned palm, I at once acquire a notion of extension,
and of the limit of that extension. The impression made by the
circular piece of gold is quite different from that which would be
made by a triangular, or a square, piece of the same size, and thereby
I arrive at the notion of figure. Moreover, if the sovereign slides
over the palm, I acquire a distinct conception of change of place
or motion, and of the direction of that motion. For as the sovereign
slides, it affects new nerve-endings, and gives rise to new states of
consciousness. Each of them is definitely and separately localized by
a reflex act of the mind, which, at the same time, becomes aware of
the difference between two successive localizations; and therefore of
change of place, which is motion.

If, while the sovereign lies on the hand, the latter being kept quite
steady, the fore-arm is gradually and slowly raised; the tactile
sensations, with all their accompaniments, remain exactly as they
were. But, at the same time, something new is introduced; namely, the
sense of effort. If I try to discover where this sense of effort seems
to be, I find myself somewhat perplexed at first; but, if I hold the
fore-arm in position long enough, I become aware of an obscure sense
of fatigue, which is apparently seated either in the muscles of the
arm, or in the integument directly over them. The fatigue seems to be
related to the sense of effort, in much the same way as the pain which
supervenes upon the original sense of contact, when a pin is slowly
pressed against the skin, is related to touch.

A little attention will show that this sense of