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.
Origins
An example of this paradox in non-western thought can be found in the origin of the Chinese word for contradiction. This term originates from a story in the 3rd century BC philosophical book HanFeizi. 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", 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, Zeusturns both creatures into static stars.
Applications
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.
Cultural references
In Iain Banks's novel, Walking on Glass, a solution to the paradox is given. The 2005 video game 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.
