place, nor shape, nor their mutual space relations as objects on the same right line, nor their physical relations to any other extra-organic objects. Yet something seems to happen to them. What? Well, the seeming is roughly described as similar to what would seem to happen to them if B did move, instead of remaining, as it does, in the right line ABC. Beyond this, however, since B changes its space-relations no whit, nothing can be said as to the happenings in the objects themselves. So far, then, there has occurred no definable happening that you can submit to verification, except the phenomenon vaguely definable as apparently similar in quality to movement. So the question still persists: What, then, has happened in the external world? We should be wholly at a loss to say, were we not able to define, just as we have already done, what has happened to the retina of the eye, viewed as a physical object. This retina has just before been exposed for a time to the influence of a seen and now objectively definable, i.e., verifiable, real movement. It is now turned upon actually resting objects. What is now seen is, therefore, definable as an after-image; and so far, indeed, one is dealing with objective happenings in the external world. The rest of the experience is indeed subjective, and whoever is curious to find what it is, must try and see.
The axiom that whatever moves must change its place, refers, therefore, only to external physical movements, and refers to them only in so far as, in order to be recognized as external, their characters must be definable as happenings of this or of this type. Could anybody tell what happens when a thing appears to move and yet does not change place, and did this described happening involve other describable physical changes than those of the retina of the eye, then the phenomenon in question would belong to physics instead of merely to psychology.
VI.
But it is time to pass from these illustrations to a more general statement of the meaning of our thesis. What I here maintain involves at once a psychological and a philosophical
