make ideas recur, they will be considered in the next proposition but one.
The method of reasoning used in the last paragraph is farther confirmed by the following circumstance; viz. that both the diminutive declining sensations, which remain for a short space after the impressions of the objects cease, and the ideas, which are the copies of such impressions, are far more distinct and vivid, in respect of visible and audible impressions, than of any others. To which it may be added, that, after travelling, hearing music, &c. trains of vivid ideas are very apt to recur, which correspond very exactly to the late impressions, and which are of an intermediate nature between the remaining sensations of the third proposition, in their greatest vigour, and the ideas mentioned in this.
The sensations of feeling, taste and smell, can scarce be said to leave ideas, unless very indistinct and obscure ones. However, as analogy leads one to suppose that these sensations may leave traces of the same kind, though not in the same degree, as those of sight and hearing; so the readiness with which we reconnoitre sensations of feeling, taste, and smell, that have been often impressed, is an evidence that they do so; and these generated traces or dispositions of mind may be called the ideas of feeling, taste, and smell. In sleep, when all our ideas are magnified, those of feeling, taste, and smell, are often sufficiently vivid and distinct; and the same thing happens in some few cases of vigilance.
This correspondence of the diminutive vibrations to the original sensory ones, consists in this, that they agree in kind, place, and line of direction; and differ only in being more feeble, i.e. in degree.
This proposition follows from the foregoing. For since sensations, by being often repeated, beget ideas, it cannot but be that those vibrations, which accompany sensations, should beget something which may accompany ideas in like manner; and this can be nothing but feebler vibrations, agreeing with the sensory generating vibrations in kind, place, and line of direction.
Or thus: By the first proposition it appears, that some motion must be excited in the medullary substance, during each sensation; by the fourth, this motion is determined to be a vibratory one: since therefore some motion must also, by the second, be excited in the medullary substance during the presence of each idea, this motion cannot be any other than a vibratory one: else how should it proceed from the original vibration attending the sensation, in the same manner as the idea does from the sensation
