sensation which I had from the spot in its original position. My world
of experience thus far has been a purely qualitative one. I might go
on to eternity having experiences of the same kind, and never dream of
space, or geometry, nor should I have the unique experience of a
geometrical illusion, either optical or tactual. Now suppose I set up
the bodily movements of the eyes or the head, or of the whole body,
which are necessary to follow the path of that point, until I overtake
it and once more restore the quality of the original sensation. This
circle, completed by the two processes of external activity and
restoration by internal activity, forms a group of sensations which
constitutes the ultimate atom in our spatial experience. I have my
first spatial experience when I have the thrill of satisfaction that
comes from overtaking again, by means of my own inner activity, a
sensation that has escaped me through an activity not my own. A being
incapable of motion, in a world of flux, would not have the spatial
experience that we have. A being incapable of motion could not make
the distinction between an outer change that can be corrected by an
internal change, and an outer change that cannot so be restored. Such
an external change incapable of restoration by internal activity we
should have if the spot on the paper changed by a chemical process
from black to red.

Now such a space theory is plainly not to be confused with the theory
that makes the reversibility of the spatial series its primary
property. It is evident that we can have a series of sensations which
may be reversed and yet not give the notion of space. But we should
always have space-perception if one half of the circular process above
described comes from an outer activity, and the other half from an
inner activity. This way of describing the reversibility of the
spatial series makes it less possible to urge against it the