Irresistible force paradox
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The irresistible force paradox, also called the unstoppable force paradox or shield and spear paradox, is a classic paradox formulated as "What happens when an unstoppable force meets an immovable object?" This paradox is a form of the omnipotence paradox, which is a simple demonstration that challenges omnipotence: "Can God create a stone so heavy that not even God is strong enough to lift it?" The immovable object and the irresistible force are both implicitly assumed to be indestructible, or else the question would have a trivial resolution ("it destroys it"). Furthermore, it is assumed that they are two separate entities, since an irresistible force is implicitly an immovable object, and vice versa.
The paradox arises because it rests on two premises—that there exist such things as irresistible forces and immovable objects—which cannot both be true at once. If there exists an irresistible force, it follows logically that there cannot be any such thing as an immovable object, and vice versa.
An example of this paradox in non-western thought can be found in the origin of the Chinese word for paradox (Chinese: 矛盾; pinyin: máodùn; literally: "Spear-Shield"). This term originates from a story (see the Kanbun example) in the 3rd century BC philosophical book Han Feizi. In the story, a man was trying to sell a spear and a shield. When asked how good his spear was, he said that his spear could pierce any shield. Then, when asked how good his shield was, he said that it could defend from all spear attacks. Then one person asked him what would happen if he were to take his spear to strike his shield; the seller could not answer. This led to the idiom of "zìxīang máodùn" (自相矛盾), or "self-contradictory". Another ancient and mythological example illustrating this theme can be found in the story of the Teumessian fox, who can never be caught, and the hound Laelaps, who never misses what it hunts. Realizing the paradox, Zeus turns both creatures into static stars.
In popular culture, Iain Banks refers to this paradox extensively in the novel Walking on Glass, and provides an alternative answer. Also in popular culture Superman resolves the issue by the simple answer "they surrender" in an issue of the non-canon story "All-Star Superman" written by Grant Morrison. In the film "The Dark Knight", The Joker portrayed by Heath Ledger taunts the Batman, asking the irresistible force paradox.
- Han Feizi (韓非子), chapter 36, Nanyi (難一 "Collection of Difficulties, No. 1")'.