Each of our special senses, therefore, may be conceived as straining towards development of a wider kind than earthly experience has as yet allowed. And each special sense is both an internal and an external sense; involves a tract of the brain, of unknown capacity, as well as an end-organ, whose capacity is more nearly measurable. The relation of this internal, mental, mind's-eye vision to non-sensory psychological perception on the one hand, and to ocular vision on the other hand, is exactly one of the points on which some profounder observation will be seen to be necessary. One must at least speak of "mind's eye" perception in these sensory terms, if one is to discuss it at all. But ordinary experience at any rate assumes that the end-organ alone can acquire fresh information, and that the central tract can but combine this new information already sent in to it. This must plainly be the case, for instance, with optical or acoustic knowledge;--with such knowledge as is borne on waves of ether or of air, and is caught by a terminal apparatus, evolved for the purpose. But observe that it is by no means necessary that all seeing and all hearing should be through eye or ear. The vision of our dreams--to keep to vision alone for greater simplicity--is non-optical vision. It is usually generated in the central brain, not sent up thither from an excited retina. Optical laws can only by a stretch of terms be said to apply to it at all. Let us attempt some rough conspectus, which may show something of the relation in which central and peripheral vision stand to each other. We start from a region below the specialisation of visual faculty. The study of the successive dermal and nervous modifications which have led up to that faculty belongs to Biology, and all that our argument needs here is to point out that the very fact that this faculty has been developed in a germ, animated by metetherial life, indicates that some perceptivity from which sight could take its origin pre-existed in the originating, the unseen world. We know vaguely how vision differentiated itself peripherally, with the growing sensibility of the pigment-spot to light and shadow. But there must have been a cerebral differentiation also, and also a psychological differentiation, namely, a gradual shaping of a distinct feeling from obscure feelings, whose history we cannot recover.