List of paradoxes
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This is a list of paradoxes, grouped thematically. Note that many of the listed paradoxes have a clear resolution once the reason behind the paradox is understood -- see Quine's Classification of Paradoxes.
Contents |
[edit] Logical (except mathematical)
- Paradox of entailment: Inconsistent premises always make an argument valid.
- Raven paradox (or Hempel's Ravens): Observing a red apple increases the likelihood of all ravens being black.
- Horse paradox: All horses are the same color.
- Unexpected hanging paradox: The day of the hanging will be a surprise, so it cannot happen at all, so it will be a surprise. The Bottle Imp paradox uses similar logic.
- Drinker paradox: In any non-empty pub, there is a customer such that, if he or she drinks, everybody in the pub drinks.
- Carroll's paradox: "Whatever Logic is good enough to tell me is worth writing down..."
- Lottery paradox: it is philosophically justifiable to believe that every individual lottery ticket won't win, but not justifiable to believe that no lottery ticket will win.
[edit] Self-referential
These form a well-known (and well-studied) class having in common a contradiction arising from self-reference.
- Berry paradox: The phrase "the first number not nameable in under ten words" appears to name it in nine words.
- Brian's paradox: "You are all individuals!" "I'm not!"
- Curry's paradox: "If this sentence is true, the world will end in a week."
- Epimenides paradox: A Cretan says "All Cretans are liars".
- Exception paradox: "If there is an exception to every rule, then every rule must have at least one exception, excepting this one" ...is there an exception to the rule that states that there is an exception to every rule?
- Grelling-Nelson paradox: Is the word "heterological", meaning "not applicable to itself," a heterological word? (Another close relative of Russell's paradox.)
- Hegel's paradox: "Man learns from history that man learns nothing from history."
- Liar paradox: "This sentence is false." This is the canonical self-referential paradox.
- The Y combinator in the lambda calculus and combinatory logic has been called the paradoxical combinator since it is related to the self-referential antinomies.
- Petronius' paradox: "Moderation in all things, including moderation."
- Quine's paradox: "yields a falsehood when appended to its own quotation" yields a falsehood when appended to its own quotation.
- Paradox of the Court
- Russell's paradox: Does the set of all those sets that do not contain themselves contain itself? Russell popularised it with the Barber paradox: The adult male barber who shaves all men who do not shave themselves, and no-one else.
- Richard's paradox: We appear to be able to use simple English to define a decimal expansion in a way which is self-contradictory.
- Shaynes Paradox: The following sentence is true. The previous sentence is false.
- Don't read this: There is no possible way to obey the statement because by the time the command is given to you it has already been contradicted.
- Intentionally blank page: Many documents contain pages on which the text "This page intentionally blank", thereby making the page not blank.
- Drasin's Paradox: "Everything is biased. If you disagree with me, it is due to a bias of your own self."
- Ordering a servant not to serve you. By not serving you he disobeys the order, as he is obeying the order not to serve you.
[edit] Vagueness
- Ship of Theseus (a.k.a. George Washington's or Grandfather's old axe): When every component of the ship has been replaced at least once, is it still the same ship?
- Sorites paradox: At what point does a heap stop being a heap as I take away grains of sand? Alternately, at what point does someone become bald?
[edit] Mathematical and statistical
- See also: Category:Mathematics paradoxes
- Apportionment paradox: Some systems of apportioning representation can have unintuitive results
- Arrow's paradox/Voting paradox You can't have all the attributes of an ideal voting system at once.
- Benford's law: In lists of numbers from many real-life sources of data, the leading digit 1 occurs much more often than the others.
- Elevator paradox: Elevators can seem to be mostly going in one direction, as if they were being manufactured in the middle of the building and being disassembled on the roof and basement.
- Inspection paradox: Why you will wait longer for that bus than you should.
- Interesting number paradox: The first number that can be considered "dull" rather than "interesting" becomes interesting because of that fact.
- Intransitive dice
- Lindley's paradox: tiny errors in the null hypothesis are magnified when large data sets are analyzed, leading to false but highly statistically significant results
- Low birth weight paradox: Low birth weight and mothers who smoke contribute to a higher mortality rate. Babies of smokers have lower average birth weight, but low birth weight babies born to smokers have a lower mortality rate than other low birth weight babies. (A special case of Simpson's paradox.)
- Missing dollar paradox: Faulty logic makes it appear as if a dollar from a restaurant bill has gone missing. Not in the same class as the others.
- Statistical paradox: It is quite possible to draw wrong conclusions from correlation. For example, towns with a larger number of churches generally have a higher crime rate — because both result from higher population. A professional organisation once found that economists with a Ph.D. actually had a lower average salary than those with a BS — but this was found to be due to the fact that those with a Ph.D. worked in academia, where salaries are generally lower. This is also called a spurious relationship.
- Will Rogers phenomenon: the mathematical concept of an average, whether defined as the mean or median, leads to apparently paradoxical results - for example, it is possible that moving an entry from Wikipedia to Wiktionary would increase the average entry length on both sites
- Wright's paradox: A baby grows old faster than an old man, as doubling of age or duration is more frequent at the beginning of the process than near the end.
[edit] Probability
- See also: Category:Probability theory paradoxes
- Berkson's paradox: a complicating factor arising in statistical tests of proportions
- Bertrand's paradox (probability): Different common-sense definitions of randomness give quite different results.
- Birthday paradox: What is the chance that two people in a room have the same birthday?
- Borel's paradox: Conditional probability density functions are not invariant under coordinate transformations.
- Boy or Girl: If in a two-child family, one child is a boy, what is the probability that the other child is a girl?
- Hodgson's paradox: the ratio of two Gaussian random variables, both with equal mean, has neither mean nor variance.
- Monty Hall problem: An unintuitive consequence of conditional probability. Essentially the same as the Three Prisoners Problem.
- Simpson's paradox: An association in sub-populations may be reversed in the population. It appears that two sets of data separately support a certain hypothesis, but, when considered together, they support the opposite hypothesis.
- Sleeping Beauty problem: A probability problem that can be correctly answered as one half or one third depending on how the question is approached.
- Three cards problem: When pulling a random card, how do you determine the color of the underside.
- Two-envelope paradox: You are given two indistinguishable envelopes and you are told one contains twice as much money as the other. You may open one envelope, examine its contents, and then, without opening the other, choose which envelope to take.
[edit] Infinity
- Burali-Forti paradox: If the ordinal numbers formed a set, it would be an ordinal number which is smaller than itself.
- Galileo's paradox: Though most numbers are not squares, there are no more numbers than squares. (See also Cantor, Diagonal Argument)
- Hilbert's paradox of the Grand Hotel: If a hotel with infinitely many rooms is full, it can still take in more guests.
- Monty Hell problem: Positive daily profits yield zero assets in the (infinite) limit.
- Skolem's paradox: Countably infinite models of set theory contain uncountably infinite sets.
- Bank and mermaid: an example showing that infinite, non convergent series can be added up to any wanted sum.
[edit] Geometry and topology
- Banach–Tarski paradox: Cut a ball into 5 pieces, re-assemble the pieces to get two balls, both of equal size to the first.
- Gabriel's Horn or Torricelli's trumpet: A simple object with finite volume but infinite surface area. Also, the Mandelbrot set and various other fractals have finite area, but infinite perimeter (in fact, there are no two distinct points on the border of the Mandelbrot set that can be reached from one another by moving a finite distance along the border, which also implies that in a sense you go no further if you walk "the wrong way" around the set to reach a nearby point).
- Hausdorff paradox: There exists a countable subset C of the sphere S such that S\C is equidecomposable with two copies of itself.
- Coastline paradox: the perimeter of a landmass is in general ill-defined
- Smale's paradox: A sphere can, topologically, be turned inside out.
- Missing square puzzle: Two similar figures have different areas while built from the same pieces.
[edit] Decision theoretic
- Abilene paradox: People can make decisions based not on what they actually want to do, but on what they think that other people want to do, with the result that everybody decides to do something that nobody really wants to do, but only what they thought that everybody else wanted to do.
- Buridan's ass: How can a rational choice be made between two outcomes of equal value?
- Control paradox: Man can never be free of control, for to be free of control is to be controlled by oneself.
- Morton's fork: Choosing between unpalatable alternatives.
- Paradox of hedonism: When one pursues happiness itself, one is miserable; but, when one pursues something else, one achieves happiness.
- Newcomb's paradox: How do you play a game against an omniscient opponent?
- Kavka's toxin puzzle: Can one intend to drink the deadly toxin, if the intention is the only thing needed to get the reward?
[edit] Chemical
- SAR paradox: Exceptions to the principle that a small change in a molecule causes a small change in its chemical behavior are frequently profound.
[edit] Physical
- Bell's spaceship paradox: concerning relativity.
- Black hole information paradox: Black holes violate a commonly assumed tenet of science - that information cannot be destroyed.
- Braess' paradox: Sometimes adding extra capacity to a network can reduce overall performance.
- Carroll's paradox: The angular momentum of a stick should be zero, but is not.
- D'Alembert's paradox: An inviscid liquid produces no drag.
- Denny's paradox: Surface-dwelling arthropods (such as the water strider) should not be able to propel themselves horizontally.
- Ehrenfest paradox: On the kinematics of a rigid, rotating disk.
- Einstein-Podolsky-Rosen paradox: Can far away events influence each other in quantum mechanics?
- Fermi paradox: If there are, as probability would suggest, many other sentient species in the Universe, then where are they? Shouldn't their presence be obvious?
- Gibbs paradox: In an ideal gas, is entropy an extensive variable?
- The GZK paradox: High-energy cosmic rays have been observed which seem to violate the Greisen-Zatsepin-Kuzmin limit which is a consequence of special relativity.
- Ladder paradox: A classic relativity problem.
- Loschmidt's paradox: Why is there an inevitable increase in entropy when the laws of physics are invariant under time reversal? The time reversal symmetry of physical laws appears to allow the second law of thermodynamics to be broken.
- Mixing paradox: On the change of entropy of a system before and after mixing.
- Mpemba paradox: Hot water can under certain conditions freeze faster than cold water, even though it must pass the lower temperature on the way to freezing.
- Olbers' paradox: Why is the night sky black if there is an infinity of stars?
- Ontological paradox
- Schrödinger's cat paradox: A quantum paradox - Is the cat alive or dead before we look?
- Supplee's paradox: the buoyancy of a relativistic object (such as a bullet) appears to change when the reference frame is changed from one in which the bullet is at rest to one in which the fluid is at rest.
- Twin paradox: When the travelling twin returns, he is younger and older than his brother who stayed put.
[edit] Philosophical
- Epicurean paradox, or Problem of evil: The existence of evil seems to be incompatible with the existence of an omnipotent and caring God.
- Free will and omniscience paradox: If there is an omniscient being then it is impossible to have free will, for the omniscient being already knows what you are going to decide, therefore you can't decide because the decision has already been made.
- Grandfather paradox: You travel back in time and kill your grandfather before he meets your grandmother which precludes your own conception and, therefore, you couldn't go back in time and kill your grandfather.
- Hutton's Paradox: If asking oneself "Am I dreaming?" in a dream proves that one is, what does it prove in waking life?
- Liberal paradox
- Mere addition paradox: Is a large population living barely tolerable lives better than a small happy population?
- Moore's paradox: "It's raining, but I don't believe that it is."
- Nihilist paradox: If truth does not exist, the statement "truth does not exist" is a truth, thereby proving itself incorrect.
- Omnipotence paradox: Can an omnipotent being create a rock too heavy to lift?
- Related to the above, the irresistible force paradox: can an irresistible force move an immovable object? (Both of these upon inspection can also be considered paradoxes of entailments.)
- Paradox of hedonism: in seeking pleasure, one does not find happiness.
- Predestination paradox: A man travels back in time and impregnates his great-great-grandmother. The result is a line of offspring and descendants, including the man's parent(s) and the man himself. Therefore, unless he makes the time-travel trip at all, he will never exist.
- Zeno's paradoxes: "You will never reach point B from point A as you must always get half-way there, and half of the half, and half of that half, and so on..."
[edit] Psychological
- Quite successful with children of a certain age is the so-called reverse psychology, where you ask one thing to achieve the opposite.
- Someone encouraging you to Be more spontaneous! or to Relax!!, thereby making it impossible.
- The "bored airline pilots"-announcements: "Remain quiet! There is no cause for alarm!" (from How to Irritate People), causing a panic amongst passengers.
- Mixed Messages occur when you say something but all non-verbal actions "say" the opposite, like a woman saying "No" to a flirting man, but smiling and using a very friendly tone of voice. In long-term relationships (especially between parents and child) this ambigous behaviour can lead to so-called Double bind-structures (as described by Gregory Bateson, like when the mother says "I love you" stone-faced, with a cold voice, and without touching the child.
- It is a generally accepted tenet of human nature that one wants that which one cannot have; conversely, when one gets that which one had wanted, frequently, one no longer wants it.
- In Modern art, some works appear ugly or even repulsive to a traditional taste, but beautiful within their own new aesthetics.
[edit] Economic
- See also: Category:Economics paradoxes
- Abilene paradox: A group of people often has to decide against its own interests.
- Allais paradox: A change in a possible outcome which is shared by different alternatives affects people's choices among those alternatives, in contradiction with expected utility theory.
- Bertrand paradox: Two players reaching a state of Nash equilibrium both find themselves with no profits.
- Bird in the bush paradox: Why are people so risk-averse?
- Diamond-water paradox (or paradox of value) Why is water cheaper than diamonds, when humans need water to survive, not diamonds?
- Edgeworth paradox: With capacity constraints, there may not be an equilibrium.
- Ellsberg paradox: People exhibit ambiguity aversion (as distinct from risk aversion), in contradiction with expected utility theory.
- Gibson's paradox: Why were interest rates and prices correlated?
- Giffen paradox: Can increasing the price of bread make poor people eat more of it?
- Jevons paradox: Increases in efficiency lead to even larger increases in demand.
- Leontief paradox: Some countries export labor-intensive commodities and import capital-intensive commodities, in contradiction with Heckscher-Ohlin theory.
- Marching Chinese paradox: "If all the Chinese in the world were to line up four abreast, I think it was, and start marching past a given point, they'd never stop because of the babies that would be born and grow up before they passed the point." (Quoted from C. Kornbluth, The Marching Morons, 1957)
- Paradox of thrift: If everyone saves more money during times of recession, then aggregate demand will fall and will in turn lower total savings in the population.
- Parrondo's paradox: It is possible to play two losing games alternately to eventually win.
- Productivity paradox, Solow computer paradox Worker productivity may go down, despite technological improvements...
- St. Petersburg paradox: People will only offer a modest fee for a reward of infinite value.
