LII. They say that the first inclination which an animal has is to protect itself, as nature brings herself to take an interest in it from the beginning, as Chrysippus affirms in the first book of his treatise on Ends; where he says, that the first and dearest object to every animal is its own existence, and its consciousness of that existence. For that it is not natural for any animal to be alienated from itself, or even to be brought into such a state as to be indifferent to itself, being neither alienated from nor interested in itself. It remains, therefore, that we must assert that nature has bound the animal to itself by the greatest unanimity and affection; for by that means it repels all that is injurious, and attracts all that is akin to it and desirable. But as for what some people say, that the first inclination of animals is to pleasure, they say what is false. For they say that pleasure, if there be any such thing at all, is an accessory only, which, nature, having sought it out by itself, as well as those things which are adapted to its constitution, receives incidentally in the same manner as animals are pleased, and plants made to flourish.
Moreover, say they, nature makes no difference between animals and plants, when she regulates them so as to leave them without voluntary motion or sense; and some things too take place in ourselves in the same manner as in plants. But, as inclination in animals tends chiefly to the point of making them pursue what is appropriate to them, we may say that their inclinations are regulated by nature. And as reason is given to rational animals according to a more perfect principle, it follows, that to live correctly according to reason, is properly predicated of those who live according to nature. For nature is as it were the artist who produces the inclination.
LIII. On which account Zeno was the first writer who, in his treatise on the Nature of Man, said, that the chief good was confessedly to live according to nature; which is to live according to virtue, for nature leads us to this point. And in like manner Cleanthes speaks in his treatise on Pleasure, and so do Posidonius and Hecaton in their essays on Ends as the Chief Good. And again, to live according to virtue is the same thing as living according to one’s experience of those things which happen by nature; as Chrysippus explains it in the first book of his treatise on the Chief Good. For our individual natures are all parts of universal nature; on which account the chief good is to live in a manner corresponding to nature, and that means corresponding to one’s own nature and to universal nature; doing none of those things which the common law of mankind is in the habit of forbidding, and that common law is identical with that right reason which pervades everything, being the same with Jupiter, who is the regulator and chief manager of all existing things.