And by parity of reason, if that species of motion which we term vibrations, can be shown, by probable arguments, to attend upon all sensations, ideas, and motions, and to be proportional to them; then we are at liberty either to make vibrations the exponent of sensations, ideas, and motions, or these the exponents of vibrations, as best suits the inquiry; however impossible it may be to discover in what way vibrations cause, or are connected with, sensations, or ideas; i.e. though vibrations be of a corporeal, sensations and ideas of a mental, nature.
If we suppose an infinitesimal elementary body to be intermediate between the soul and gross body, which appears to be no improbable supposition, then the changes in our sensations, ideas, and motions, may correspond to the changes made in the medullary substance, only as far as these correspond to the changes made in the elementary body. And if these last changes have some other source besides the vibrations in the medullary substance, some peculiar original properties, for instance, of the elementary body, then vibrations will not be adequate exponents of sensations, ideas, and motions. Other suppositions to the same purpose might be made; and upon the whole, I conjecture, that though the first and second propositions are true, in a very useful practical sense, yet they are not so in an ultimate and precise one.
The most vigorous of our sensations are termed sensible pleasures and pains, as noted above, in the Introduction. And the vivid nature of these engages us to be very attentive to their several properties, relations, and oppositions. It is requisite therefore, in our inquiry into the doctrine of vibrations, to examine, how far the phænomena of sensible pleasure and pain can be deduced from, or explained by, it.
First, then, the doctrine of vibrations seems to require, that each pain should differ from the corresponding and opposite pleasure, not in kind, but in degree only; i.e. that pain should be nothing more than pleasure itself, carried beyond a due limit. For of the four differences of vibrations mentioned in the first corollary of the foregoing proposition, three are given, viz. those of kind, place, and line of direction, in the pleasures and pains which correspond, as opposites to each other: there is therefore nothing left, from whence the difference of such pleasures and pains can arise, except the difference of degree. But the phænomena appear to be sufficiently suitable to this reasoning, inasmuch as all pleasure appears to pass into pain, by increasing its cause, impression, duration, sensibility of the organ upon which it is impressed, &c. Thus an agreeable warmth may be made to pass into a troublesome or burning heat, by
