The duck test is a form of abductive reasoning. This is its usual expression: 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.
History
poet James Whitcomb Riley may have coined the phrase when he wrote: 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: 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: Later references to the duck test include Cardinal Richard Cushing's, who used the phrase in 1964 in reference to Fidel Castro. Douglas Adams parodied this test in his book Dirk Gently's Holistic Detective Agency: Monty Python also referenced the test in the Witch Logic scene in their 1975 film Monty Python and the Holy Grail: A claimed recent application of the duck test was the denial of tax exempt "nonprofit" status to Blue Shield of California: The Liskov Substitution Principle in computer science is sometimes expressed as a counter-example to the duck test: Russian Minister of Foreign AffairsSergey 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: Professor Vladimir Vapnik, a pioneer and co-inventor of Support Vector Machines and a major contributor to the theory ofmachine 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.
Elephant test
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 opined "like the elephant, it is characterised more by recognition when encountered than by definition." Overruling in part R v Ghosh. A similar incantation was invoked by the concurring opinion of Justice Potter Stewart in Jacobellis v. Ohio, 378 U.S. 184, 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."
