As a dog returneth to his vomit, so a fool returneth to his folly.
As a dog returneth to his vomit - quoted in 2 Peter 2:22, and so stamped with inspired authority. As a dog swallows again that which he has vomited.
(So) a fool returneth to his folly. A dog is an unclean animal, and so is a fit image of the fool, whose delight is in sin. Though the dog has experienced the meat so unwholesome that his stomach rejected it, yet, now that it has become still more so by exposure to sun and air, he goes back to it; so the fool.