In a certain sect, there was an alchemist who once boasted, "My alchemical skills are excellent and renowned throughout Shenzhou. I shall provide alchemical services for all within the Immortal Dao who cannot concoct their potions. I will only serve them!" Those who came to him for alchemy were naturally those who couldn't concoct their own potions. However, one day, feeling that his cultivation had stagnated, he instinctively opened his pill furnace. Then, could he concoct a potion for himself?
This is the famous Alchemist's Paradox. When reduced to a low-version without magic, it's still the Alchemist's Paradox. If one prefers a more sophisticated expression, it is also equivalent to, "Russell's Paradox."
