The sensible motion of a sensum in its field really is something absolute; it does not simply consist in the fact that this sensum alters its spatial relations to other sensa in the field, though, of course, it involves this as a necessary consequence. Thus, sensible motion is the way in which the positional non-uniformity of a four-dimensional object presents itself to all when this non-uniformity is "sharp" enough to be noticeable within the duration of a single sense-field. It is very important to notice that the movement of sensa in their fields and of sense-objects in the spaces of their sense-histories is the ultimate empirical basis of the concept of absolute motion. There are also certain points to be noticed about the shapes, of the successive sensa in the sense-object. While the observer keeps his head still, the successive sensa will be indistinguishable in shape, unless, of course, physical changes are going on in the object.