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.[1]


An example of this paradox in western thought can be found in the origin of the Chinese word for contradiction (Chinese: 矛盾; pinyin: máodùn; literally: 'spear-shield'). This term originates from a story in the 3rd century BC philosophical book Han Feizi.[2] 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 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.[3]


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.[4]

In the same way, an unstoppable force, an object or force with infinite inertia, would be consistent with the definition of an immovable object, in that they would be one and the same. Any object whose momentum or motion cannot be changed is an immovable object, and it would halt any object that moved relative to it, making it an unstoppable force.[5]

A deterministic universe may contain more than one of such forces/objects as long as they are never determined to meet in the entire history of such a universe. Indeed, in the context of such a universe, one could redefine the words "irresistible" and "immovable" to "is never successfully resisted" and "is never successfully moved" (within the fixed history of said deterministic universe) instead of the counterfactual possibilities. This is similar to the Novikov self-consistency principle of the grandfather paradox in time-travel scenarios.

Cultural references[edit]

In Iain Banks's novel, Walking on Glass, a solution to the paradox is given.

In DC Comics' All-Star Superman by Grant Morrison and Frank Quitely, Superman encounters the Ultrasphinx, who asks "What happens when the unstoppable force meets the immovable object?", to which Superman answers "They surrender."

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 Knight Rider, KITT references the paradox when he was faced with the possibility of having a Head-on collision with his prototype version called KARR, who has most of KITT's abilities including a nearly indestructible body.

In DC's film The Dark Knight (2008), the Joker references the irresistible force paradox in both his final scene, as well as his final dialogue between himself and Batman. The Joker attempts to ultimately explain the reason for Batman's inability to shed his Batman identity, saying, "You just couldn't let me go, could you? This is what happens when an unstoppable force meets an immovable object. You truly are incorruptible, aren't you, huh? You won't kill me out of some misplaced sense of self-righteousness, and I won't kill you because you're just too much fun. I think you and I are destined to do this forever."


  1. ^ Mike Alder (2004). "Newton's Flaming Laser Sword". Philosophy Now. 46: 29–33.
    Also available as Mike Alder (2004). "Newton's Flaming Laser Sword" (PDF). Mike Alder's Home Page. University of Western Australia. Archived from the original (PDF) on November 14, 2011.
  2. ^ Han Feizi (韓非子), chapter 36, Nanyi (難一 "Collection of Difficulties, No. 1")'.
  3. ^ DK Publishing (2012). Nature Guide Stars and Planets. Penguin. p. 275. ISBN 978-1-4654-0353-7.
  4. ^ Kaczor, Christopher (2009). This Rock, 20(3).
  5. ^ minutephysics (2013-01-25), Immovable Object vs. Unstoppable Force - Which Wins?, retrieved 2017-12-11