Paradox

Logically self-contradictory statement

For other uses, see Paradox (disambiguation).

A paradox is a logically self-contradictory statement or a statement that runs contrary to one's expectation.[1][2] It is a statement that, despite apparently valid reasoning from true or apparently true premises, leads to a seemingly self-contradictory or a logically unacceptable conclusion.[3][4] A paradox usually involves contradictory-yet-interrelated elements that exist simultaneously and persist over time.[5][6][7] They result in "persistent contradiction between interdependent elements" leading to a lasting "unity of opposites".[8]

In logic, many paradoxes exist that are known to be invalid arguments, yet are nevertheless valuable in promoting critical thinking,[9] while other paradoxes have revealed errors in definitions that were assumed to be rigorous, and have caused axioms of mathematics and logic to be re-examined. One example is Russell's paradox, which questions whether a "list of all lists that do not contain themselves" would include itself and showed that attempts to found set theory on the identification of sets with properties or predicates were flawed.[10][11] Others, such as Curry's paradox, cannot be easily resolved by making foundational changes in a logical system.[12]

Examples outside logic include the ship of Theseus from philosophy, a paradox that questions whether a ship repaired over time by replacing each and all of its wooden parts one at a time would remain the same ship.[13] Paradoxes can also take the form of images or other media. For example, M. C. Escher featured perspective-based paradoxes in many of his drawings, with walls that are regarded as floors from other points of view, and staircases that appear to climb endlessly.[14]

Informally, the term paradox is often used to describe a counterintuitive result.

## Common elements

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Self-reference, contradiction and infinite regress are core elements of many paradoxes.[15] Other common elements include circular definitions, and confusion or equivocation between different levels of abstraction.

### Self-reference

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Self-reference occurs when a sentence, idea or formula refers to itself. Although statements can be self referential without being paradoxical ("This statement is written in English" is a true and non-paradoxical self-referential statement), self-reference is a common element of paradoxes. One example occurs in the liar paradox, which is commonly formulated as the self-referential statement "This statement is false".[16] Another example occurs in the barber paradox, which poses the question of whether a barber who shaves all and only those who do not shave themselves will shave himself. In this paradox, the barber is a self-referential concept.

### Contradiction

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Contradiction, along with self-reference, is a core feature of many paradoxes.[15] The liar paradox, "This statement is false," exhibits contradiction because the statement cannot be false and true at the same time.[17] The barber paradox is contradictory because it implies that the barber shaves himself if and only if the barber does not shave himself.

As with self-reference, a statement can contain a contradiction without being a paradox. "This statement is written in French" is an example of a contradictory self-referential statement that is not a paradox and is instead false.[15]

### Vicious circularity, or infinite regress

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Vicious circularity illustrated

Another core aspect of paradoxes is non-terminating recursion, in the form of circular reasoning or infinite regress.[15] When this recursion creates a metaphysical impossibility through contradiction, the regress or circularity is vicious. Again, the liar paradox is an instructive example: "This statement is false"—if the statement is true, then the statement is false, thereby making the statement true, thereby making the statement false, and so on.[15][18]

The barber paradox also exemplifies vicious circularity: The barber shaves those who do not shave themselves, so if the barber does not shave himself, then he shaves himself, then he does not shave himself, and so on.

### Other elements

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Other paradoxes involve false statements and half-truths or rely on hasty assumptions (A father and his son are in a car crash; the father is killed and the boy is rushed to the hospital. The doctor says, "I can't operate on this boy. He's my son." There is no contradiction, the doctor is the boy's mother.).

Paradoxes that are not based on a hidden error generally occur at the fringes of context or language, and require extending the context or language in order to lose their paradoxical quality. Paradoxes that arise from apparently intelligible uses of language are often of interest to logicians and philosophers. "This sentence is false" is an example of the well-known liar paradox: it is a sentence that cannot be consistently interpreted as either true or false, because if it is known to be false, then it can be inferred that it must be true, and if it is known to be true, then it can be inferred that it must be false. Russell's paradox, which shows that the notion of the set of all those sets that do not contain themselves leads to a contradiction, was instrumental in the development of modern logic and set theory.[10]

Thought experiments can also yield interesting paradoxes. The grandfather paradox, for example, would arise if a time traveler were to kill his own grandfather before his mother or father had been conceived, thereby preventing his own birth. This is a specific instance of the butterfly effect - in that any interaction a time traveler has with the past would alter conditions such that divergent events "propagate" through the world over time, ultimately altering the circumstances in which the time travel initially takes place.

Often a seemingly paradoxical conclusion arises from an inconsistent or inherently contradictory definition of the initial premise. In the case of that apparent paradox of a time traveler killing his own grandfather, it is the inconsistency of defining the past to which he returns as being somehow different from the one that leads up to the future from which he begins his trip, but also insisting that he must have come to that past from the same future as the one that it leads up to.

## Quine's classification

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W. V. O. Quine (1962) distinguished between three classes of paradoxes:[19][20]

### Veridical paradox

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See also: Veridicality

A veridical paradox produces a result that appears counter to intuition, but is demonstrated to be true nonetheless:

* The fact that the Earth is an approximately spherical object that is rotating and in rapid motion around the Sun contradicts the apparently obvious and common-sensical appearance of the Earth as a stationary approximately flat plane illuminated by a Sun that rises and falls throughout the day.
* Condorcet's paradox demonstrates the surprising result that majority rule can be self-contradictory, i.e. it is possible for a majority of voters to support some outcome other than the one chosen (regardless of the outcome itself).
* The Monty Hall paradox (or equivalently three prisoners problem) demonstrates that a decision that has an intuitive fifty–fifty chance can instead have a provably different probable outcome. Another veridical paradox with a concise mathematical proof is the birthday paradox.
* In 20th-century science, Hilbert's paradox of the Grand Hotel or the Ugly duckling theorem are famously vivid examples of a theory being taken to a logical but paradoxical end.
* The divergence of the harmonic series: ∑ n = 1 ∞ 1 n = 1 + 1 2 + 1 3 + 1 4 + 1 5 + ⋯ . {\displaystyle \sum _{n=1}^{\infty }{\frac {1}{n}}=1+{\frac {1}{2}}+{\frac {1}{3}}+{\frac {1}{4}}+{\frac {1}{5}}+\cdots .}

### Falsidical paradox

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A falsidical paradox establishes a result that appears false and actually is false, due to a fallacy in the demonstration. Therefore, falsidical paradoxes can be classified as fallacious arguments:

* The various invalid mathematical proofs are classic examples of this, like the ones that attempt to prove that 1=2, which often rely on an inconspicuous division by zero.
* The horse paradox, which falsely generalises from true specific statements
* Zeno's paradoxes are falsidical, concluding, for example, that a flying arrow never reaches its target or that a speedy runner cannot catch up to a tortoise with a small head-start.

### Antinomy

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An antinomy is a paradox which reaches a self-contradictory result by properly applying accepted ways of reasoning. For example, the Grelling–Nelson paradox points out genuine problems in our understanding of the ideas of truth and description.

Sometimes described since Quine's work, a dialetheia is a paradox that is both true and false at the same time. It may be regarded as a fourth kind, or alternatively as a special case of antinomy. In logic, it is often assumed, following Aristotle, that no dialetheia exist, but they are allowed in some paraconsistent logics.

## Ramsey's classification

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Frank Ramsey drew a distinction between logical paradoxes and semantic paradoxes, with Russell's paradox belonging to the former category, and the liar paradox and Grelling's paradoxes to the latter.[21] Ramsey introduced the by-now standard distinction between logical and semantical contradictions. Logical contradictions involve mathematical or logical terms like class and number, and hence show that our logic or mathematics is problematic. Semantical contradictions involve, besides purely logical terms, notions like thought, language, and symbolism, which, according to Ramsey, are empirical (not formal) terms. Hence these contradictions are due to faulty ideas about thought or language, and they properly belong to epistemology.[22]

## In medicine

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A paradoxical reaction to a drug is the opposite of what one would expect, such as becoming agitated by a sedative or sedated by a stimulant. Some are common and are used regularly in medicine, such as the use of stimulants such as Adderall and Ritalin in the treatment of attention deficit hyperactivity disorder (also known as ADHD), while others are rare and can be dangerous as they are not expected, such as severe agitation from a benzodiazepine.[23]

The actions of antibodies on antigens can rarely take paradoxical turns in certain ways. One example is antibody-dependent enhancement (immune enhancement) of a disease's virulence; another is the hook effect (prozone effect), of which there are several types. However, neither of these problems is common, and overall, antibodies are crucial to health, as most of the time they do their protective job quite well.

In the smoker's paradox, cigarette smoking, despite its proven harms, has a surprising inverse correlation with the epidemiological incidence of certain diseases.

## See also

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* Philosophy portal

* Absurdism – Theory that life in general is meaningless
* Animalia Paradoxa – Mythical, magical or otherwise suspect animals mentioned in Systema Naturae
* Aporia – State of puzzlement or expression of doubt, in philosophy and rhetoric
* Contronym
* Dilemma – Problem requiring a choice between equally undesirable alternatives
* Ethical dilemma – Type of dilemma in philosophy
* Fallacy – Argument that uses faulty reasoning
* Formal fallacy – Faulty deductive reasoning due to a logical flaw
* Four-valued logic – Any logic with four truth values
* Impossible object – Type of optical illusion
* Category:Mathematical paradoxes
* List of paradoxes – List of statements that appear to contradict themselves
* Mu (negative) – Term meaning 'not', 'without', or 'lack'
* Oxymoron – Figure of speech
* Paradox of tolerance – Logical paradox in decision-making theory
* Paradox of value – Contradiction between utility and price
* Paradoxes of material implication – Type of logical contradiction
* Plato's beard – Example of a paradoxical argument
* Revision theory
* Self-refuting idea – Idea that refutes itself
* Syntactic ambiguity – Sentences with structures permitting multiple possible interpretations
* Temporal paradox – Theoretical paradox resulting from time travel
* Twin paradox – Thought experiment in special relativity
* Zeno's paradoxes – Set of philosophical problems

## References

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### Notes

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1. ^ Weisstein, Eric W. "Paradox". mathworld.wolfram.com. Retrieved 2019-12-05.
2. ^ "By “paradox” one usually means a statement claiming something that goes beyond (or even against) ‘common opinion’ (what is usually believed or held)." Cantini, Andrea; Bruni, Riccardo (2017-02-22). "Paradoxes and Contemporary Logic". In Zalta, Edward N. (ed.). Stanford Encyclopedia of Philosophy (Fall 2017 ed.).
3. ^ "paradox". Oxford Dictionary. Oxford University Press. Archived from the original on February 5, 2013. Retrieved 21 June 2016.
4. ^ Bolander, Thomas (2013). "Self-Reference". The Metaphysics Research Lab, Stanford University. Retrieved 21 June 2016.
5. ^ Smith, W. K.; Lewis, M. W. (2011). "Toward a theory of paradox: A dynamic equilibrium model of organizing". Academy of Management Review. 36 (2): 381–403\. doi:10.5465/amr.2009.0223. JSTOR 41318006.
6. ^ Zhang, Y.; Waldman, D. A.; Han, Y.; Li, X. (2015). "Paradoxical leader behaviors in people management: Antecedents and consequences" (PDF). Academy of Management Journal. 58 (2): 538–566\. doi:10.5465/amj.2012.0995.
7. ^ Waldman, David A.; Bowen, David E. (2016). "Learning to Be a Paradox-Savvy Leader". Academy of Management Perspectives. 30 (3): 316–327\. doi:10.5465/amp.2015.0070. S2CID 2034932.
8. ^ Schad, Jonathan; Lewis, Marianne W.; Raisch, Sebastian; Smith, Wendy K. (2016-01-01). "Paradox Research in Management Science: Looking Back to Move Forward" (PDF). Academy of Management Annals. 10 (1): 5–64\. doi:10.5465/19416520.2016.1162422. ISSN 1941-6520.
9. ^ Eliason, James L. (March–April 1996). "Using Paradoxes to Teach Critical Thinking in Science". Journal of College Science Teaching. 15 (5): 341–44\. Archived from the original on 2013-10-23.
10. ^ a b Irvine, Andrew David; Deutsch, Harry (2016), "Russell's Paradox", in Zalta, Edward N. (ed.), The Stanford Encyclopedia of Philosophy (Winter 2016 ed.), Metaphysics Research Lab, Stanford University, retrieved 2019-12-05
11. ^ Crossley, J.N.; Ash, C.J.; Brickhill, C.J.; Stillwell, J.C.; Williams, N.H. (1972). What is mathematical logic?. London-Oxford-New York: Oxford University Press. pp. 59–60\. ISBN 0-19-888087-1. Zbl 0251.02001.
12. ^ Shapiro, Lionel; Beall, Jc (2018), "Curry's Paradox", in Zalta, Edward N. (ed.), The Stanford Encyclopedia of Philosophy (Summer 2018 ed.), Metaphysics Research Lab, Stanford University, retrieved 2019-12-05
13. ^ "Identity, Persistence, and the Ship of Theseus". faculty.washington.edu. Retrieved 2019-12-05.
14. ^ Skomorowska, Amira (ed.). "The Mathematical Art of M. C. Escher". Lapidarium notes. Retrieved 2013-01-22.
15. ^ a b c d e Hughes, Patrick; Brecht, George (1975). Vicious Circles and Infinity - A Panoply of Paradoxes. Garden City, New York: Doubleday. pp. 1–8\. ISBN 0-385-09917-7. LCCN 74-17611.
16. ^ S.J. Bartlett; P. Suber (2012). Self-Reference: Reflections on Reflexivity (illustrated ed.). Springer Science & Business Media. p. 32\. ISBN 978-94-009-3551-8. Extract of page 32
17. ^ C.I. Lewis: The Last Great Pragmatist. SUNY Press. 2005. p. 376\. ISBN 978-0-7914-8282-7. Extract of page 376
18. ^ Myrdene Anderson; Floyd Merrell (2014). On Semiotic Modeling (reprinted ed.). Walter de Gruyter. p. 268\. ISBN 978-3-11-084987-5. Extract of page 268
19. ^ Quine, W.V. (1966). "The ways of paradox". The Ways of Paradox, and other essays. New York: Random House. ISBN 9780674948358.
20. ^ W.V. Quine (1976). The Ways of Paradox and Other Essays (REVISED AND ENLARGED ed.). Cambridge, Massachusetts and London, England: Harvard University Press.
21. ^ Fraser MacBride; Mathieu Marion; María José Frápolli; Dorothy Edgington; Edward Elliott; Sebastian Lutz; Jeffrey Paris (2020). "Frank Ramsey". Chapter 2. The Foundations of Logic and Mathematics, Frank Ramsey, < Stanford Encyclopedia of Philosophy>. Metaphysics Research Lab, Stanford University.
22. ^ Cantini, Andrea; Riccardo Bruni (2021). "Paradoxes and Contemporary Logic". Paradoxes and Contemporary Logic (Fall 2017), <Stanford Encyclopedia of Philosophy>. Metaphysics Research Lab, Stanford University.
23. ^ Wilson MP, Pepper D, Currier GW, Holloman GH, Feifel D (February 2012). "The Psychopharmacology of Agitation: Consensus Statement of the American Association for Emergency Psychiatry Project BETA Psychopharmacology Workgroup". Western Journal of Emergency Medicine. 13 (1): 26–34\. doi:10.5811/westjem.2011.9.6866. PMC 3298219. PMID 22461918.

### Bibliography

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* Bjørdal, Frode (2012). "Librationist closures of the paradoxes". Logic and Logical Philosophy. 21 (4): 323–361\. doi:10.12775/LLP.2012.016. hdl:10852/24479. ISSN 1425-3305.
* Sainsbury, R. M. (2009) [1987]. Paradoxes (3rd ed.). Cambridge: Cambridge University Press. ISBN 978-0-521-89632-0. OCLC 244652614.
* Poundstone, William (2011) [1989]. Labyrinths of Reason: Paradox, Puzzles, and the Frailty of Knowledge. Westminster: Knopf Doubleday Publishing Group. ISBN 978-0-385-24271-4.
* Sorensen, Roy A. (2003). A Brief History of the Paradox: Philosophy and the Labyrinths of the Mind. Oxford; New York: Oxford University Press. ISBN 978-0-19-515903-5.
* Hughes, Patrick (2011). Paradoxymoron: Foolish Wisdom in Words and Pictures. London: Reverspective. ISBN 978-0-956-80610-9.

## External links

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Wikiquote has quotations related to Paradox.

Look up paradox in Wiktionary, the free dictionary.

Wikimedia Commons has media related to Paradoxes.

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* Cantini, Andrea (Winter 2012). "Paradoxes and Contemporary Logic". In Zalta, Edward N. (ed.). Stanford Encyclopedia of Philosophy.
* Spade, Paul Vincent (Fall 2013). "Insolubles". In Zalta, Edward N. (ed.). Stanford Encyclopedia of Philosophy.
* "Zeno and the Paradox of Motion". MathPages.com.
* ""Logical Paradoxes"". Internet Encyclopedia of Philosophy.
* Smith, Wendy K.; Lewis, Marianne W.; Jarzabkowski, Paula; Langley, Ann (2017). The Oxford Handbook of Organizational Paradox. Oxford University Press. ISBN 9780198754428.

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