Of course to some extent the answer depends on what we mean by Sensation. If by that term we intend our whole Experience of the external, then of course it necessarily follows--or, at least, we admit--that our Knowledge of the external must be thence derived. But such a use of the term is loose, misleading, and infrequent. The only safe course is to confine the term Sensation to the immediate data of the five senses--touch, sight, hearing, smell, and taste, with probably the addition of muscular and other internal feelings. It is in this sense that the word is usually employed, and has been employed by the Sensationalist School themselves. Now we might perhaps begin by taking the idea of Time as a concept constantly employed in Discourse, but of which it would be absurd to suggest that it is supplied to us by Sensation. It might, however, be urged in reply that the idea of Time is not derived from the external world at all, but is furnished to us directly by the operations of the Mind, and that therefore its intellectual origin need not involve any exception to the general rule that the materials of our Knowledge of the world are furnished by Sensation alone. Without, therefore, entering upon any discussion of the interesting question as to what is the real nature of Time, we shall pass to the idea of Space. Mach, the writer whom we have already quoted, in his essay on _Space and Geometry_ speaks constantly and freely of sensations of Space, and as there can be no denial of the fact that Space is a constituent of the external world, it would seem to follow that those who hold Sensation to be the only source of our Knowledge must be obliged to affirm the possibility of sensations of Space. Mach indeed claims to distinguish physiological Space, geometrical Space, visual Space, tactual Space as all different and yet apparently harmoniously blended in our Experience. He is, however, sadly wanting in clearness of statement. He never tells us when and where exactly we do have a sensation of Space. In truth he never gets behind the postulate of an all-enveloping tridimensional world; so that he throughout assumes Space as a datum, and his inquiry is an effort to rediscover Space where he has already placed it. Let us, however, consider for a moment what can be meant by a sensation of Space. Does it not look very like a contradiction in terms? Pure Space, if it means anything, means absolute material emptiness and vacuity. How, then, by any possibility can it give rise to a sensation? What sensory organ can it be conceived as affecting? How and in what way can it be felt?