On n'a jamais vu de chien faire, de propos délibéré, l'échange d'un os avec un autre chien.
Contexte et précisions
En anglais : "Nobody ever saw a dog make a fair and deliberate exchange of one bone for another with another dog. Nobody ever saw one animal by its gestures and natural cries signify to another, this is mine, that yours; I am willing to give this for that....But man has almost constant occasion for the help of his brethren, and it is in vain for him to expect it from their benevolence only."