It is obvious, however, that two dimensions are only an abstraction from three, and that the spatial relations of bodies, considered fully, involve three dimensions; in other words, spatial characteristics are, properly speaking, three-dimensional. It follows that terms which fully state spatial characteristics can never express what things look, but only what they are. A body may be cylindrical, and we may see a cylindrical body; but such a body can never, strictly speaking, _look_ cylindrical. The opposition, however, between what a thing _is_ and what it _looks_ implies that what it _is_ is independent of a percipient, for it is precisely correlation to a percipient which is implied by 'looking' or 'appearing'. In fact, it is the view that what a thing really is it is, independently of a percipient, that forms the real starting-point of Kant's thought. It follows, then, that the spatial characteristics of things, and therefore space itself, must belong to what they are in themselves apart from a percipient, and not to what they look.[28] Consequently, it is so far from being true that we only know what things look and not what they are, that in the case of spatial relations we actually know what things are, even though they never look what they are. [28] This consideration disposes of the view that, if colour is relative to perception, the primary qualities, as being inseparable from colour, must also be relative to perception; for it implies that the primary qualities cannot from their very nature be relative to perception. Moreover, if the possibility of the separation of the primary qualities from colour is still doubted, it is only necessary to appeal to the blind man's ability to apprehend the primary qualities, though he may not even know what the word 'colour' means. Of course, it must be admitted that some sensuous elements are involved in the apprehension of the primary qualities, but the case of the blind man shows that these may relate to sight instead of to touch. Moreover, it, of course, does not follow from the fact that sensuous elements are inseparable from our perception of bodies that they belong to, and are therefore inseparable from, the bodies perceived.