The _physiology_ of a sense, consists in an external organ,--as the eye or ear, its nerves of sensation which spread through the brain, and, the nervous fluid. To each of the senses there belongs a distinct cerebral organ, which, if deducted, leaves nothing to constitute the physiology, but the external organ, the nerves, and nervous fluid; such may be considered the physiology of all the senses, so far as the exciting mental perception is concerned. The _function of a sense_ is, to act on and excite the cerebral organ, when the nervous fluid is put into an acting state through external circumstances. The _result of the function_, is a sensation, of which we have immediate cognizance, by reason of a sensation being _a recent change in consciousness_. The nervous fluid, not the tubular nervous _striæ_, is that by which the brain is excited. The _use of the sensation_ is manifold. Emanating from the wonderful Economy, is the law, that, _the sensation which an external body promotes, shall, to ourself, seem to belong to that body_. The law is imperative. The sensation being apparently at, and belonging to, the external object or body, it is imagined the body is visible, seen by the eyes, and of the colour, flavour, or odour known by the sensation. The apparent place of the sensation directs to where the body is situated. No person thinks, when a rose promotes the sensation of colour, that the object perceived is within himself: without the sensation there is no perception of red, and with it, nothing is perceived or seen of colour or of the flower; so that, were the object coloured or not, it is to the spectator invisible; and as the sensation would be useless were the object coloured and seen, it is obvious that the flower is uncoloured, therefore is not seen: the seeing an uncoloured object is a physical absurdity. So is it with all sensations; they constitute the only objects of perception with which we are acquainted; and, such as they are in any respect, the outward objects are in no respect. Sound is a sensation; a sense has been provided that we should have knowledge of sound; there is nothing of sound or noise in the air; the function of the sense is not to hear, but excite the auditory cerebral organ, and the sensation, in which alone sound consists, _seems_ to be outside of us, and _seems_ to come from a bell, but which has nothing of the kind to part with; yet it is imagined that sound enters the ear. Thus is it supposed that the sensation externally exists, and is sound heard by the ear. The philosopher so instructed, calculates the velocity of the physical nonentity sound.