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.
When I see a bird that walks like a duck and swims like a duck and quacks like a duck, I call that bird a duck.
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.
[Bedevere:] There are ways of telling whether she is a witch! [Villagers:] Are there? What? Tell us, then! Tell us! [B:] Tell me. What do you do with witches? [Vs:] Burn! Burn! You burn them! Burn! [B:] And what do you burn apart from witches? [Vs:] More witches! Wood! [B:] So, why do witches burn? [Villager:] 'Cos they're made of wood? [B:] Good! ... So; how do we tell if she is made of wood? [V:] Build a bridge out of 'er! [B:] Ah, but can you not also make bridges out of stone? [Vs:] Oh yeah. [B:] Does wood sink in water? [Vs:] No, it floats! Throw her into the pond! Yaa! [B:] What also floats in water? [Vs:] Bread! Apples! Very small rocks? Cider! Gra-Gravy! Cherries! Mud! Churches? Churches! Lead! Lead! [King Arthur:] A duck! [Vs:] Ooh! [B:] Exactly. So, logically... [V:] If she weighs the same as a duck, she's made of wood... [B:] and therefore... [V:] a witch!
In 2015, a variation of the duck test was applied in the revocation of tax exempt "nonprofit" status to Blue Shield of California:
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.
The Liskov Substitution Principle in computer science is sometimes expressed as a counter-example to the duck test:
Russian Minister of Foreign Affairs Sergey Lavrov used a version of the Duck Test in 2015 in response to allegations that Russian airstrikes in Syria were not targeting terrorist groups, primarily ISIS, but rather West-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?
Professor Vladimir Vapnik, a pioneer and co-inventor of Support Vector Machines (SVM) and a major contributor to the theory of machine learning and many foundational ideas in statistical 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 humorousness of this line lies in its violation of an expected opposite.
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." Overruling in part 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
- Duck typing – A 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
- Philosophical razor – Principle that allows one to eliminate unlikely explanations
- Sympathetic magic – Type of magic based on imitation or correspondence
- Zebra (medicine) – Unnecessarily exotic diagnosis in medicine
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- 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)
- Christy, Howard Chandler; Ethel Franklin Betts (1982), The complete works of James Whitcomb Riley