I can by a simple illustration show more plainly what I mean by the statement that the blending of the inner and outer sensations is necessary for the perception of space.  I shall use the sense of sight for the illustration, although precisely the same reasoning would apply to the sense of touch.  Suppose that I sat in an entirely passive position and gazed at a spot on an otherwise blank piece of paper before me.  I am perfectly passive so far as motion on my part is concerned.  I may be engaged in any manner of speculation or be in the midst of the so-called active attention to the spot; but I must be and for the present remain motionless.  Now, while I am in this condition of passivity, suppose the spot be made to move slowly to one side by some force external to myself.  I am immovable all the while, and yet am conscious of this movement of the spot from the first position, which I call A, to the new position, A’, where it stops.  The sensation which I now have is qualitatively different from the 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 objections that Stumpf[20] has formulated