Let us then distinguish in the human passions, the physical from the moral; that is, the cause from the effect. The first emotion is in the internal material sense; this the mind may receive but cannot produce. Let us likewise distinguish momentary from durable emotions, and we shall immediately perceive, that fear, horror, rage, love, or rather the desire of enjoyment, are sensations which, though durable, depend solely on the impressions of objects upon our senses, combined with the remaining impressions of our preceding sensations; and that, of consequence, those passions we enjoy in common with the brutes. I mention the actual impressions of objects, as being combined with the impressions that remain of our former sensations, for neither to man nor beast nothing is horrible, nor attractive, when seen for the first time. Of this we have proof in young animals, who will run into the fire the first time it is presented to them. By reiterated acts, of which the impressions subsist in their internal sense, do they alone acquire experience; and though this experience is not natural, it is not less sure, and is even on that account more circumspect. A violent motion, a great noise, an extraordinary figure, which is seen or heard suddenly, and for the first time, produces in the animal a shock of which the effect is similar to the first movements of fear. But this sentiment is only instantaneous; for as it cannot be combined with any preceding sensation, so it must communicate to the animal a transitory vibration, and not a durable emotion, such as the passion of fear supposes. A young and peaceful tenant of the forests, who suddenly hears the sound of the huntsman's horn, or the report of a gun, leaps, bounds, and flies off, by the sole violence of the shock which it has experienced. Yet if this noise is without effect and ceases, the animal distinguishing the wonted silence of Nature, composes itself, halts, and returns to its tranquil retreat. But age and experience render it circumspect and timid, and having been wounded after a particular noise, the sensation of pain is retained in its internal sense, and when the same noise shall be again heard, it is renewed, combines itself with the actual agitation, and produces a permanent passion, a real fear; the animal flies with all its might, and frequently never returns to its usual abode.