There is very little ground, either from reason or experience, to conclude the universe eternal or incorruptible. The continual and rapid motion of matter, the violent revolutions with which every part is agitated, the changes remarked in the heavens, the plain traces as well as tradition of a universal deluge,—all these prove strongly the mortality of this fabric of the world, and its passage, by corruption or dissolution, from one state or order to another. It must therefore, as well as each individual form which it contains, have its infancy, youth, manhood, and old age; and it is probable that in all these variations man, equally with every animal and vegetable, will partake. In the flourishing age of the world it may be expected that the human species should possess greater vigour both of mind and body, more prosperous health, higher spirits, longer life, and a stronger inclination and power of generation. But if the general system of things, and human society of course, have any such gradual revolutions, they are too slow to be discernible in that short period which is comprehended by history and tradition. Stature and force of body, length of life, even courage and extent of genius, seem hitherto to have been naturally in all ages pretty much the same. The arts and sciences, indeed, have flourished in one period and have decayed in another; but we may observe that at the time when they rose to greatest perfection among one people they were perhaps totally unknown to all the neighbouring nations, and though they universally decayed in one age, yet in a succeeding generation they again revived and diffused themselves over the world. As far, therefore, as observation reaches there is no universal difference discernible in the human species, and though it were allowed that the universe, like an animal body, had a natural progress from infancy to old age; yet, as it must still be uncertain whether at present it be advancing to its point of perfection or declining from it, we cannot thence presuppose any decay in human nature.​[40] To prove, therefore, or account for the greater populousness of antiquity by the imaginary youth or vigour of the world will scarcely be admitted by any just reasoner; these general physical causes ought entirely to be excluded from that question.