"Do you see the cat? She sits there, and will lick her paws and keep herself clean." "I see that," said the other. "Well," said the first speaker, "Did you ever hear of one of the hogs taken out of the pigsty that ever acted like the cat?" "No," he said. "But he could if he wanted to," said the other. "Yes! truly he could if he wanted to; but it is not according to his nature, and you never saw such a thing done, and until you have changed the swine's nature, he cannot perform such a good action, and God's Word says the same thing of man."

-- Charles Haddon Spurgeon (1834-1892)

(Thanks to Stuart Johnson for the reference)