instance, the objects A, B, and C existing as a matter of physical fact on a right line, and they will then remain on a right line so long as you look. But if one of these objects, say B, falls on the portion of the field of vision where the after-image of the motion occurs, you will meanwhile see B moving, yet the right line ABC will seem to be undistorted and unbroken, although A and C seem to rest, and B to move. This paradox, as noticed by Fleischl in a brief paper in the Wiener Sitzungsberichte (Bd. 86, 4.1), wherein attention was first called, I believe, to just this aspect of the after-images of motion, provoked from Fleischl himself the somewhat ill-humored remark that our sensations are so irrational as not to be subject to the principle of contradiction. But one must not be ill-humored with our sensations. They have their own impredictable wealth. Movement without change of place, and without distortion of perceived space relations to surrounding objects, is, as these experiments show, neither self-contradictory nor otherwise impossible in the inner life. But in the external world such movements appear to be excluded a priori. Why? Because a real movement, if it is to be definitely verified by all observers, must first be describable as an external happening in a definite and general way before one can undertake to verify it. But one describes or defines an external happening in general terms by mentioning either its conditions or its results, both of these being definable in terms of space relations amongst the things concerned. It is true that the experiment with these subjective after-images of motion can indeed be described, can indeed be tried and so verified by many others; but what none of these observers can define is what really seemed to them to happen to and in the seen outer objects themselves at the moment when they thus paradoxically seemed at once to move and to rest. For these objects, e.g., B, did not change form or content or place, in any definable way. Hence, no observer can point out to his fellow, in terms of preceding or succeeding space relations, what change in the seen objects he refers to when he says that they seem to him to move. There remains the incommunicable and therefore
