o follow that those who hold Sensation to
be the only source of our Knowledge must be obliged to affirm the
possibility of sensations of Space. Mach indeed claims to distinguish
physiological Space, geometrical Space, visual Space, tactual Space as
all different and yet apparently harmoniously blended in our Experience.
He is, however, sadly wanting in clearness of statement. He never tells
us when and where exactly we do have a sensation of Space. In truth he
never gets behind the postulate of an all-enveloping tridimensional
world; so that he throughout assumes Space as a datum, and his inquiry
is an effort to rediscover Space where he has already placed it.

Let us, however, consider for a moment what can be meant by a sensation
of Space. Does it not look very like a contradiction in terms? Pure
Space, if it means anything, means absolute material emptiness and
vacuity. How, then, by any possibility can it give rise to a sensation?
What sensory organ can it be conceived as affecting? How and in what way
can it be felt?

The truth is the idea of Space is essentially negative. It represents
absence of physical obstruction of every kind. No doubt, we may describe
it positively as a possibility of free movement, and such a description
is at once true and important. Yet even _it_ involves a negative. The
term "free" is in reality, though not in form, a negative term and means
"unconstrained." And the reason why such a term is necessarily negative
is to be found in the fact that a state of dynamic constraint is the
essential condition under which we enter upon our organic existence.
Freedom is a negation of the Actual. Absolute freedom is a condition
only theoretically possible, and is essentially the negation of the
state of restraint in which our life is maintained.

But the definition last quoted is nevertheless valuable because it
clearly shows what really is the origin of the idea of Space. It proves
that the idea of Space is a representation of one condition of our
Activity. It is because the primary work of Thought is to represent the
forms of our dynamic Activity that we find the idea of Space so
necessary and fundamental.

But it will perhaps be argued that our ordinary sensations carry with
them a spatial meaning and implication, and that indirectly, therefore,
our sensations _do_ supply us with the idea of Space. It will readily be
agreed that if this is so of any sensations it is pre-eminently true of
the s