Duck test

The duck test is a form of abductive reasoning, usually expressed as "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. Douglas Adams parodied this test in his book Dirk Gently's Holistic Detective Agency: If it looks like a duck, and quacks like a duck, we have at least to consider the possibility that we have a small aquatic bird of the family Anatidae on our hands. 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, since ducks float just as wood floats, and wood burns just as witches burn, then a person who weighs the same as a duck must be a witch. The Liskov Substitution Principle in computer science is sometimes expressed as a counter-example to the duck test: If it looks like a duck and quacks like a duck but it needs batteries, you probably have the wrong abstraction. 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.