_What_, then, is truly meant, when it is said, that, for producing the mental affection, which constitutes hearing, some previous vibration is necessary? It certainly cannot mean, as I have already remarked, that the vibration is any thing in itself different from the series of physical events which it expresses, however few or numerous these may be, since it is only the name which we give to them, when we consider them together; nor can it mean that the direct cause of the sensation is any thing different from the one organic state immediately preceding the sensation,--a state which may, indeed, have resulted from a long sequence of prior organic states, produced during the continued vibratory motion of the air, but which is itself, in its relation to the phenomenon which succeeds it,--that affection of the sentient mind which constitutes _hearing_,--to be considered independently of these prior states, that have no other relation to the mind, than as gradually inducing that ultimate organic state, which is the state that is followed by sensation. There is a part, less or greater, of the sensorial organ, which must be affected, in a certain manner, before the sensation of hearing can take place; and, in _vibration_, there is nothing but a repeated approach and secession of the vibrating particles. If _vibration_, then, or a series of pulses, be necessary, it is evident that a corresponding series of changes in the organ is necessary; that is to say, there is no one instant, at which the vibrating particles are in such a state relatively to the sensorial organ, that if no previous change had been excited in the organ itself, they could have produced in it immediately, the precise state, which is instantly followed by the mental affection of hearing. There must, therefore, be a _series_ of changes, in the sensorial organ itself, the last of which only is followed by sensation. The particles of the air, or any other elastic medium, for example, must, in their _first_ appulse, produce a certain state of the sensorial organ; in their _second_ appulse, a different state, by acting on an organ, already affected in a certain manner; in their _third_ appulse, a still different state; and thus successively, till, _at last_, they produce that particular definite state of the sensorial organ, in consequence of which, the _mind_ becomes instantly _sentient_,--a state which could not have been produced by any single impulse of the particles on the unaffected organ, because then vibration, or a series of pulses, would not have been necessary.