There was once upon a time a certain rabbi who was deeply respected by all the people of his community. His words seemed to them wisdom itself, and they felt guided and uplifted by his teaching. Nevertheless, there was one person. a man named Isaac, who always challenged the rabbi’s interpretations of the law, and pounced upon any perceived error in his reasoning. The other members of the community found his behaviour tedious and disrespectful, and they often grumbled about Isaac, but he paid them no attention, continuing to argue with the rabbi at every opportunity.

Then, one day, as must happen to us all, Isaac passed away. When the congregation gathered for the funeral, they could see that the rabbi looked unusually sad.

“Rabbi,” they asked him, “why are you so sad? Are you sorrowing for Isaac? He was a trouble-maker! A stone in your path! You should thank God that he is gone.”

The rabbi sighed, and said, “Yes, I am sad. It is true that Isaac was a stone, stubborn and dense and unmoving, but a knife needs a stone to sharpen it. I do not sorrow for Isaac – his soul is in heaven now – but I sorrow for myself. Without the stone of Isaac, how shall I keep myself sharp?”