The fable of the Sheath and the Knife (Diderot)

One day, the Sheath and the Knife began to quarrel. The Knife said to the Sheath:

"Sheath, my love, you are a fickle jade, for every day you accommodate new knives."

The Sheath answered the Knife: "Knife, my dear, you are a faithless knave, for every day you change sheaths."

"Sheath, that is not what you promised me."

"Knife, you deceived me first."

This quarrel occurred at table. The man sitting between the Sheath and the Knife spoke up saying:

"You, Sheath, and you, Knife, were both right to change, since change is what suits the both of you. But you were quite wrong to promise that you would not change. Knife, can you not see that God made you to fit more than one Sheath? And you, Sheath, to accommodate more than one Knife? You thought certain knives were mad when they vowed to dispense altogether with sheaths, and you thought certain sheaths mad when they vowed to remain closed to all knives. But you did not think that you were just as mad as they when you, Sheath, swore to limit yourself to one Knife, and you, Knife, never to look beyond one Sheath..."

Denis Diderot, Jacques the Fatalist