have the sense of smell, and perhaps that of taste in greater perfection than we have. There will therefore be still less room left for the variety of intercourses between the optic and auditory nerves in the medullary substance of the brain. And yet it is evident, from obvious observations, as well as from the whole tenor of these papers, that the eye and ear, with their associations, are the chief sources of intellect; and that the greatest part of the pleasures and pains of human life arise from visible and audible impressions, which in themselves afford neither pleasure nor pain.
Thus it is natural to expect, that the happiness and misery of brutes should depend principally, and in a direct manner, on the impressions made upon their gross senses, whilst that of mankind arises, in great measure, from long trains of associated ideas and emotions, which enter chiefly by the eye and ear. And it seems to me a very striking coincidence, that mankind should at the same time exceed the brute creation in the variety of their ideas, and in the proportional largeness of that part of the body which is the peculiar seat of these.
The same proportional largeness may, as it were, detain the vibrations which ascend from external impressions up to the brain, and so prevent that freedom of descent into the muscular system which takes place in brutes; and which disposes them to move more early, and more readily, in consequence of direct impressions, than men, at the same time that they have a far less command, in respect of voluntary motion. But this difference depends, in great measure, upon the considerations that follow, as will be seen.
Secondly, That the very constitution and texture of the nervous system, in its infinitesimal vessels, should differ in brutes from that of men, appears highly reasonable to be expected. And since the lives of brutes fall, in general, far short of that of man, also since the quadrupeds (which resemble man more than other animals) are far more hairy, and fowls have feathers, it appears probable, that the texture of the nervous system in brutes should tend more to callosity, and fixedness, in its dispositions to vibrate, than in men. The brains of young brute animals will therefore be sooner able to retain miniatures than those of children, as tending more to firmness and fixedness in their ultimate texture and constitution; at the same time that this texture will unfit them for receiving a variety. To which, if we add the shortness of their lives, and consequently of their ascent to the summit of adult age; which ascent is the proper time for receiving instruction; it is easy to see, that on this double account, as well as that mentioned under the foregoing head, they must fall far short of mankind in the number of their intellectual ideas, pleasures, and pains.
It follows from the same method of reasoning, that the few dispositions to miniature vibrations, which are generated in
