have the power of converting original pains into pleasures, and pleasures into mere sensations, i.e. into evanescent pleasures; as we find it has in fact. For this may be effected by such a change in the organ and brain, as that the organ shall send weaker and weaker vibrations perpetually to the brain, upon every successive renewal of the same impression, and the brain become perpetually less and less disposed to receive strong vibrations, though the power of communication from the impressions should continue the same. It remains therefore to be inquired, what general tendencies in the small medullary particles might dispose them to undergo such changes. And it appears to me, that a change of the spheres of attraction and repulsion in these particles, upon every change in their situations, so as always to lean towards the situation last superinduced, might be sufficient for this purpose. However, this is a mere supposition, and that of a very recluse nature. Only let it be observed, that the fact to be here accounted for, viz. the decrease in the efficacy of impressions frequently repeated, is both an evident one, and also must have its rise from some powers in the small parts of matter over each other. It must therefore admit of an explanation, either from the doctrine of vibrations, or from some other law of matter and motion. And if the doctrine of vibrations be found suitable to other phænomena, it may be presumed not to be unsuitable to this, till such time as some manifest inconsistency between them shall appear.
It ought to be remarked here, that this transit of original pains into pleasures, and of vivid pleasures into faint ones, by frequent repetition, bears some relation to the above-mentioned transition of pains into positive local pleasures, of the same kind with themselves.
To this head of consideration may be referred Dr. Jurin’s observation; viz. “That when we have been for some time affected with one sensation, as soon as we cease to be so affected, a contrary sensation is apt to arise in us, sometimes of itself, and sometimes from such causes as at another time would not produce that sensation at all, or at least not to the same degree.” For the continued impression of the same object will so fix upon the sensory nerve, and region of the brain corresponding thereto, a tendency to one peculiar sort of vibrations, that an impression of an opposite or very different sort, must do more than usual violence to the brain; i.e. will excite a glaring sensation of an opposite nature. See the instances mentioned by Dr. Jurin, in his Essay on distinct and indistinct Vision.
Sixthly, We may account for the different kinds and degrees of pleasure and pain, from the four differences of vibrations mentioned above, viz. those of degree, kind, place, and line of direction, and their various combinations with each other. For it is obvious to conceive, that these combinations may be sufficiently numerous and distinct from each other, to answer to the
