The second part begins, "Are temperate in Youth;" the third begins, "Are prudent in their Age;" the fourth begins, "The fourth part of their life." Herein is contained the purpose of this part in general, with regard to which it is desirable to know that each effect, inasmuch as it is an effect, receives the likeness of its cause in proportion as it is capable of retaining it.
Wherefore, since our life, as has been said, and also the life of every living creature here below, is caused by Heaven, Heaven is revealed in all such effects as these, not, indeed, with the complete circle, but with part of it, in them.

Thus its movement must be not only with them, but beyond them, and as one arch of life retains (and I say retains, not only of men, but also of other living creatures) almost all the lives, ascending and descending, they must be, as it were, similar in appearance to the form of the arch.

Returning, then, to our course of life which at present we are seeking to understand, I say that it proceeds after the manner of this arch, ascending and descending.

And it is to be known that the ascent of this arch should be equal to its descent, if the material of the seed from which we spring, so complex in its nature, did not impede the law of Human Nature.

But since the humid root is of better quality more or less, and stronger to endure in one effect more than in another, being subject to the nutriment of the heat, which is our life, it happens that the arch of the life of one man is of less or of greater extent than that of another, life being shortened by a violent death or by some accidental injury; but that which is called natural by the people is that span of which it is said by the Psalmist, "Thou settest up a boundary which it is not possible to pass." And since the Master among those here living, Aristotle, had perception of this arch of which we now speak, and seems to be of opinion that our life should be no other than one ascent and one descent, therefore he says, in that chapter where he treats of Youth and of Old Age, that Youth is no other than an increase of life.