In considering the phenomena of the sense of hearing, to which I now proceed, I may apply to them the same remark, which has been already applied to the phenomena of the senses before considered. They are classed by us, as sensations, merely in consequence of our previous belief in the existence of those external bodies, the motion of which we have known to be followed by similar feelings. Our mind begins suddenly to exist in a certain state; and we call this state joy or sorrow, without supposing that it depends on the immediate presence of any external object. It begins again to exist, in a different state, and we say, that we _hear a flute_, referring the feeling immediately to an external cause. But there can be no doubt, that, in making this reference, in the one case, and not in the other, we are influenced by experience, and by experience alone. If we suppose ourselves endowed with the single sense of hearing, and incapable therefore of having previously seen or felt the flute, which is breathed before us, or any other extended and resisting object whatever, we may imagine the mere sound to recur, innumerable times, without discovering any mode by which it can give us _more_ knowledge, than we should receive from a similar recurrence of any internal joy or sorrow. That we should be able to refer it to a body, such as we now mean, when we speak of a flute, is manifestly impossible; since this implies knowledge of _solidity_, and _form_, and _colour_, which could not be acquired without touch and sight. But there seems even no reason to think, that we should refer it to any external cause whatever, unless, indeed, such a reference necessarily accompanied every feeling, which we know is far from being the case, since we have many internal pleasures, not more like to each other, than they are to the sound of a flute, which we do not refer to any thing, separate or separable, from the constitution of our own mind. In _hearing_, therefore, as in taste and smell, we do not derive from its sensations our knowledge of things external, but, in consequence of our knowledge of things external, we regard these feelings, as sensations, in the common philosophic meaning of that term. Simple as our sense of hearing may seem, it affords a striking specimen of that almost infinite variety, which is not inconsistent with the closest resemblance; and the notion which we may form of the innumerable varieties of sound, is perhaps not more vast, when we attempt to wander over its boundless discrepancies, than when we limit ourselves to its greatest similarities, in a single word of a language, or, in that which we might be inclined at first to regard as simplicity itself, a single _musical tone_.