24 And yet there are many kinds of animals, which, when they have lost their fellow, will not mate again, and spend their time as it were in solitary life; many too live on simple herbs, and will not quench their thirst except at a pure stream; one can also often see dogs refrain from food forbidden them, so that they close their famishing mouths if restraint is bidden them. Must men then be warned against that wherein brutes have learned not to transgress?
Source: Letters (New Advent)