The duck test is a form of abductive reasoning. This is its usual expression:
If it looks like a duck, swims like a duck, and quacks like a duck, then it probably is a duck.
The test implies that a person can identify an unknown subject by observing that subject's habitual characteristics. It is sometimes used to counter abstruse arguments that something is not what it appears to be.
The French automaton maker Jacques de Vaucanson created a mechanical duck in 1738. The mechanical duck would quack, move its head to eat grain which it would appear to digest, and after a short time would excrete a mixture that looked and smelled like duck droppings. The irony is that while the phrase is often cited as proof of abductive reasoning, it is not proof, as the mechanical duck is still not a living duck.
In popular culture
Monty Python also referenced the test in the Witch Logic scene in their 1975 film Monty Python and the Holy Grail, where Sir Bedevere reasons that if ducks float and wood burns, then a person who weighs the same as a duck is made of wood and should be burned as a witch.
The Liskov Substitution Principle in computer science is sometimes expressed as a counter-example to the duck test:
Vladimir Vapnik, co-inventor of the support-vector machine and a major contributor to the theory of machine learning, uses the duck test as a way to summarize the importance of simple predicates to classify things. During the discussion he often uses the test to illustrate that the concise format of the duck test is a form of intelligence that machines are not capable of producing.
The philosopher Slavoj Žižek has cited the Marx Brothers' rewording of the duck test: "He may look like an idiot and talk like an idiot, but don't let that fool you. He really is an idiot." The humor of this line lies in its violation of an expected opposite.
A common variation of the wording of the phrase may have originated much later with Emil Mazey, secretary-treasurer of the United Auto Workers, at a labor meeting in 1946 accusing a person of being a Communist:
I can't prove you are a Communist. But when I see a bird that quacks like a duck, walks like a duck, has feathers and webbed feet and associates with ducks—I'm certainly going to assume that he is a duck.
The term was later popularized in the United States by Richard Cunningham Patterson Jr., United States ambassador to Guatemala in 1950 during the Cold War, who used the phrase when he accused Guatemala's Jacobo Arbenz Guzmán government of being Communist. Patterson explained his reasoning as follows:
Suppose you see a bird walking around in a farm yard. This bird has no label that says 'duck'. But the bird certainly looks like a duck. Also, he goes to the pond and you notice that he swims like a duck. Then he opens his beak and quacks like a duck. Well, by this time you have probably reached the conclusion that the bird is a duck, whether he's wearing a label or not.
In a startling blow to one of California's biggest health insurers, the state has revoked the tax-exempt status of Blue Shield of California, forcing the company to pay tens of millions of dollars in back taxes and unleashing a torrent of calls for it to return billions of dollars to customers. The tax board's action 'was an acknowledgment of what Blue Shield was already doing, or not doing,' said Anthony Wright, head of Health Access California, a consumer advocacy group. 'And if it looks like a duck and talks like a duck, it should be taxed like a duck.'
Also in 2015, Russian Minister of Foreign Affairs Sergey Lavrov used a version of the test in response to allegations that Russian airstrikes in Syria were not targeting terrorist groups, primarily ISIS, but rather Western-supported groups such as the Free Syrian Army. When asked to elaborate his definition of "terrorist groups", he replied:
If it looks like a terrorist, if it acts like a terrorist, if it walks like a terrorist, if it fights like a terrorist, it's a terrorist, right?
In 2021, a version of the test was used by Singapore's Minister of Finance Lawrence Wong in response to claims by members of the Progress Singapore Party that their parliamentary motion on free trade agreements was not racist. He said:
But look, if it looks like a duck, if it walks like a duck, if it quacks like a duck, it is a duck.
Similarly, the term elephant test refers to situations in which an idea or thing, "is hard to describe, but instantly recognizable when spotted".
The term is often used in legal cases when there is an issue which may be open to interpretation, such as in the case of Cadogan Estates Ltd v Morris, when Lord Justice Stuart-Smith referred to "...the well-known elephant test. It is difficult to describe, but you know it when you see it", and in Ivey v Genting Casinos, when Lord Hughes (in discussing dishonesty) opined "...like the elephant, it is characterised more by recognition when encountered than by definition." This decision partially overruled R v Ghosh.
A similar incantation (used however as a rule of exclusion) was invoked by the concurring opinion of Justice Potter Stewart in Jacobellis v. Ohio, 378 U.S. 184 (1964), an obscenity case. He stated that the Constitution protected all obscenity except "hard-core pornography". Stewart opined, "I shall not today attempt further to define the kinds of material I understand to be embraced within that shorthand description; and perhaps I could never succeed in intelligibly doing so. But I know it when I see it, and the motion picture involved in this case is not that."
- Call a spade a spade – Figurative expression
- Duck typing – Style of dynamic typing in object-oriented programming
- Extensionality – Logic principle
- I know it when I see it – Colloquial expression
- Identity of indiscernibles – Impossibility for separate objects to have all their properties in common
- Inductive reasoning – Method of logical reasoning
- Operational definition – Defining a concept in terms of specific, replicable actions or procedures
- Philosophical razor – Principle that allows one to eliminate unlikely explanations
- Sympathetic magic – Type of magic based on imitation or correspondence
- The purpose of a system is what it does – Systems thinking heuristic
- Zebra (medicine) – Unnecessarily exotic diagnosis in medicine
- WP:DUCK, a Wikipedia page specifically related to the duck test
- Brian, Edwards (26 Feb 2015). "Did you know that the phrase "if it looks like a duck..." was originally about a mechanical pooing duck?". Mirror (UK newspaper). Retrieved 25 July 2021.
- Adams, Douglas (1987). Dirk Gently's Holistic Detective Agency.
- Monty Python (1975). Monty Python and the Holy Grail.
- Bailey, Derick. "SOLID Development Principles – In Motivational Pictures".
- Vladimir Vapnik: Statistical Learning an MIT Artificial Intelligence (AI) Podcast
- Sentinel, John (September 29, 1946). "Communist Expose The Case of the Duck". Milwaukee (WI) Sentinel.
- Immerman, Richard H. (1982), The CIA in Guatemala: The Foreign Policy of Intervention, Austin, Texas: University of Texas Press, p. 102
- Denver, Joseph; Ethel Franklin Betts (1965), Cushing of Boston: A Candid Portrait
- Platt, Suzy (1992). Respectfully quoted. Library of Congress, Congressional Research Service. ISBN 978-0-88029-768-4. "Attributed to Richard Cardinal Cushing. Everett Dirksen and Herbert V. Prochnow, Quotation Finder, p. 55 (1971). Unverified."
- Seipel, Tracy (March 19, 2015). "California drops hammer on Blue Shield tax-exempt status". San Jose Mercury News. Retrieved March 19, 2015.
- Melvin, Don; Cullinane, Susannah; Tawfeeq, Mohammed (October 1, 2015). "Russia's Lavrov on Syria targets: 'If it looks like a terrorist, walks like a terrorist ...'". CNN. Retrieved 9 July 2016.
- "Minister Lawrence Wong's Closing Remarks on The Parliamentary Motion on "Securing Singaporeans' Jobs and Livelihoods" on 14 September 2021". Ministry of Finance (Singapore). 14 September 2021. Retrieved 19 October 2021.
- Valuing and Judging Partners — Beyond the Elephant Test! Archived 2013-06-03 at the Wayback Machine, Edge International Review, Summer 2006
- B.Wedderburn, The Worker and the Law (3rd ed, Harmondsworth, Penguin,1986), 116.
- Catherine Barnard, The Personal Scope of the Employment Relationship Archived 2013-01-26 at the Wayback Machine, in T.Araki and S.Ouchi (eds), The Mechanism for establishing and Changing Terms and Conditions of Employment/The Scope of Labor Law and the Notion of Employees, The Japan Institute for Labour Policy and Training Report, 2004, vol.1, 131-136.
- Cadogan Estates Ltd v Morris; EWCA Civ 1671 (4 November 1998) (at paragraph 17)
- Ivey v Genting Casinos (UK) Ltd t/a Crockfords;  UKSC 67 (25 Oct 2017) (at paragraph 48)