Irresistible force paradox
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The unstoppable force paradox, also called the irresistible force paradox, shield and spear paradox, is a classic paradox formulated as "What happens when an unstoppable force meets an immovable object?" The immovable object and the unstoppable force are both implicitly assumed to be indestructible, or else the question would have a trivial resolution. Furthermore, it is assumed that they are two entities.
The paradox arises because it rests on two incompatible premises—that there can exist simultaneously such things as unstoppable forces and immovable objects. The "paradox" is flawed because if there exists an unstoppable 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 contradiction (Chinese: 矛盾; pinyin: máodùn; lit.: 'spear-shield'). This term originates from a story 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, "from each-other spear shield"), or "self-contradictory".
Another ancient 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.
The problems associated with this paradox can be applied to any other conflict between two abstractly defined extremes that are opposite.
One of the answers generated by seeming paradoxes like these is that there is no contradiction – that there is a false dilemma. Dr. Christopher Kaczor suggested that the need to change indicates a lack of power rather than the possession thereof, and as such a person who was omniscient would never need to change their mind – not changing the future would be consistent with omniscience rather than contradicting it.
The 2005 video game Phoenix Wright: Ace Attorney retells the story of the spear and shield from Han Feizi during its fifth case. A "King of Prosecutors" trophy that homages the story, depicting a cracked shield and a broken halberd, becomes an important piece of evidence during the case's events.
In the 2008 superhero film The Dark Knight, the Joker makes the statement on Batman’s daring and finally successful attempt at capturing him, by quoting “This is what happens when an unstoppable force meets an immovable object”. The quote has become iconic amongst fans of the film series.
In the Bing Crosby song "Something's Gotta Give" the opening line paraphrases the paradox by saying "When an irresistible force such as you meets an old immovable object like me"
- Mike Alder (2004). "Newton's Flaming Laser Sword". Philosophy Now. 46: 29–33.
- Han Feizi (韓非子), chapter 36, Nanyi (難一 "Collection of Difficulties, No. 1")'.
- DK Publishing (2012). Nature Guide Stars and Planets. Penguin. p. 275. ISBN 978-1-4654-0353-7.
- Kaczor, Christopher (2009). This Rock, 20(3).