XXII. And as for the third argument, it is convicted by itself, as being derived only from an unsound system of questioning proceeding from the assertions originally made; for in truth it does not necessarily follow that a thing, all the parts of which are liable to corruption, is likewise perishable itself; but this is only inevitably true of that thing of which all the parts are perishable when taken collectively and together in the same place and at the same time, since in the case of a person who has the tip of his finger cut off, he is not disabled from living, but if he had the whole collection of all his parts and limbs cut off at once, he would die immediately. Therefore in the same manner, if all the elements of the world together were all to disappear at one and the same moment, then it would be necessary to admit that the world was liable to corruption and destruction; but if each of these elements separately only changes its nature so as to assimilate to that of its nature, it is then rendered immortal rather than destroyed, according to the philosophical statement of the tragic poet-

"Nought that has once existed dies,

Though often what has been combined

Before, we separated find,

Invested with another form."

For it is the greatest folly imaginable to estimate the antiquity of the human race from the state of art; for if any one were to follow the absurdity of such a system of reasoning as this, he will prove the world to be very young indeed, and to have been made scarcely a thousand years, since all those men whom we have heard of traditionally as the discoverers in different branches of science do not go back to a greater number of years than that which I have mentioned. But if we must speak of the arts as co-eval with the race of mankind, then we must speak, drawing our arguments from natural history, and not inconsiderately or carelessly. And what is this history? The destruction of the things on the earth, not all together, but of the greatest number of them, is attributed to two principal causes, the indescribable violence and power of fire and water. And they say that each of these elements attacks them in its turn, after very long periods of revolving years. When, therefore, a conflagration seizes upon things, a stream of ethereal fire being poured down from above is frequently diffused over them, overrunning many districts of the habitable world; and when a deluge draws down the whole of the rainy nature of water, the regular rivers and torrents overflowing, and not only that, but even far exceeding the ordinary measure of a common flood. Accordingly, when the greater part of mankind is destroyed in the manners above mentioned, besides an infinity of other ways of less power and importance, it follows of necessity that the arts also must fail, for it cannot be possible to discuss science by itself without some one to reduce it to method and practice. But when those common pestilences relax their fury, and when the human race begins again to recover vigour and to flourish, descending from those who have not been previously destroyed by the evils which pressed upon them, then the arts also begin again to exist, not indeed as they were at first, but in thinner numbers from the diminution of the numbers of those who practise them. I have now then set forth to the best of my ability what I have been able to learn or to understand concerning the indestructibility of the world.