Letters 121.24

No wonder that living things are born with a gift whose absence would make birth useless. This is the first equipment that Nature granted them for the maintenance of their existence—the quality of adaptability and self-love. They could not survive except by desiring to do so. Nor would this desire alone have made them prosper, but without it nothing could have prospered. In no animal can you observe any low esteem, or even any carelessness, of self. Dumb beasts, sluggish in other respects, are clever at living. So you will see that creatures which are useless to others are alert for their own preservation. Farewell.

Seneca, Letters to Lucilius 121.24, translated by Richard Mott Gummere