The story is told of a certain learned Jew, very pious in his way, who would fast and inflict all sorts of torments upon himself, wearing sackcloth underneath his clothes.
One day, he went to visit a great tzaddik.
He wanted the tzaddik to know how pious he was, so he opened his upper garment slightly so that the sacking would be visible.
The tzaddik looked at him for a while and then said, “How clever he is! How clever he is!”
He repeated this several times until the man had to ask, “Who is so clever?”
The tzaddik answered, “The yetzer harah (‘evil inclination’), who took a man such as yourself and put him into a sack!”
–Rabbi Adin Steinsaltz