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The Turing test is a proposal for a test of a machine's capability to perform human-like conversation. Described by Alan Turing in the 1950 paper "Computing machinery and intelligence", it proceeds as follows: a human judge engages in a natural language conversation with two other parties, one a human and the other a machine; if the judge cannot reliably tell which is which, then the machine is said to pass the test. It is assumed that both the human and the machine try to appear human. In order to keep the test setting simple and universal (to explicitly test the linguistic capability of the machine instead of its ability to render words into audio), the conversation is usually limited to a text-only channel such as a teletype machine as Turing suggested or, more recently IRC or instant messaging.


The test was inspired by a party game known as the "Imitation Game", in which a man and a woman go into separate rooms, and guests try to tell them apart by writing a series of questions and reading the typewritten answers sent back. In this game, both the man and the woman aim to convince the guests that they are the woman. Turing proposed a test employing the imitation game as follows: "We now ask the question, 'What will happen when a machine takes the part of A in this game?' Will the interrogator decide wrongly as often when the game is played like this as he does when the game is played between a man and a woman? These questions replace our original, 'Can machines think?'" (Turing 1950) Later in the paper he suggested an "equivalent" alternate formulation involving a judge conversing only with a computer and a man.

Turing originally proposed the test in order to replace the emotionally charged and (for him) meaningless question "Can machines think?" with a more well-defined one. The advantage of the new question, he said, was that it "drew a fairly sharp line between the physical and intellectual capacities of a man." (Turing 1950)

Objections and replies

Turing himself suggested several objections which could be made to the test. Below are some of the objections and replies from the article in which Turing first proposed the test.

  1. Theological Objection: This states that thinking is a function of man's immortal soul and therefore a machine could not think. Turing replies by saying that he sees no reason why it would not be possible for God to grant a computer a soul if He so wished.
  2. "'Heads in the Sand' Objection: 'The consequences of machines thinking would be too dreadful. Let us hope and believe that they cannot do so.' " The objection commits the naturalistic fallacy, confusing what should not be with what cannot be.
  3. Mathematical Objections: This objection uses mathematical theorems, such as Gödel's incompleteness theorem, to show that there are limits to what questions a computer system based on logic can answer. Turing suggests that humans are too often wrong themselves and pleased at the fallibility of a machine.
  4. Mechanical Objections: A sufficiently fast machine with sufficiently large memory could be programmed with a large enough number of human questions and human responses to deliver a human answer to almost every question, and a vague random answer to the few questions not in its memory. This would simulate human response in a purely mechanical way. Psychologists have observed that most humans have a limited number of verbal responses.
  5. Data Processing Objection: Machines process data bit by bit. Humans process data holistically. In this view, even if a machine appears human in every way, to treat it as human is to indulge in anthropomorphic thinking.
  6. Argument From Consciousness: This argument, suggested by Professor Jefferson Lister states, "not until a machine can write a sonnet or compose a concerto because of thoughts and emotions felt, and not by the chance fall of symbols, could we agree that machine equals brain". Turing replies by saying that we have no way of knowing that any individual other than ourselves experiences emotions, and that therefore we should accept the test.
  7. Lady Lovelace Objection: One of the most famous objections, it states that computers are incapable of originality. This is largely because, according to Ada Lovelace, machines are incapable of independent learning. Turing contradicts this by arguing that Lady Lovelace's assumption was affected by the context from which she wrote, and if exposed to more contemporary scientific knowledge, it would become evident that the brain's storage is quite similar to that of a computer. Turing further replies that computers could still surprise humans, in particular where the consequences of different facts are not immediately recognizable.
  8. Informality of Behaviour: This argument states that any system governed by laws will be predictable and therefore not truly intelligent. Turing replies by stating that this is confusing laws of behaviour with general rules of conduct, and that if on a broad enough scale (such as is evident in man) machine behaviour would become increasingly difficult to predict. (Later research on recursive algorithms has found that, in any case, deterministic systems are capable of a chaotic diversity of behaviour.)
  9. Extra-sensory perception: Turing seems to suggest that there is evidence for extra-sensory perception. However he feels that conditions could be created in which this would not affect the test and so may be disregarded.

Discussion of relevance

There has been some controversy over which of the alternate formulations of the test Turing intended. (Moor, 2003) The term "Turing Test" is usually taken to indicate a test in which a human judge converses with a human and a computer without knowing which is which.

It has been argued that the Turing test so defined cannot serve as a valid definition of machine intelligence or "machine thinking" for at least three reasons:

  1. A machine passing the Turing test may be able to simulate human conversational behaviour, but this may be much weaker than true intelligence. The machine might just follow some cleverly devised rules. A common rebuttal in the AI community has been to ask, "How do we know humans don't just follow some cleverly devised rules?" Two famous examples of this line of argument against the Turing test are John Searle's Chinese room argument and Ned Block's Blockhead argument.
  2. A machine may very well be intelligent without being able to chat like a human.
  3. Many humans that we'd probably want to consider intelligent might fail this test (e.g., the young or the illiterate). On the other hand, the intelligence of fellow humans is almost always tested exclusively based on their utterances.

Another potential problem, related to the first objection above, is that even if the Turing test is a good operational definition of intelligence, it may not indicate that the machine has consciousness, or that it has intentionality. Perhaps intelligence and consciousness, for example, are such that neither one necessarily implies the other. In that case, the Turing test might fail to capture one of the key differences between intelligent machines and intelligent people. Of course, machines passing the test would most likely vehemently disagree.

In the words of science popularizer Larry Gonick, "I personally disagree with this criterion, on the grounds that a simulation is not the real thing."

These criticisms are directed to the Turing Test so defined, but other interpretations of Turing's "new question" have been discussed. Sterret argues that two distinct tests can be extracted from Turing's 1950 paper, and that, pace Turing's remark, they are not equivalent. The test that employs the party game and compares frequencies of success in the game is referred to as the "Original Imitation Game Test" whereas the test consisting of a human judge conversing with a human and a machine is referred to as the "Standard Turing Test". Sterrett agrees that the Standard Turing Test (STT) has the problems its critics cite, but argues that, in contrast, the Original Imitation Game Test (OIG Test) so defined is immune to many of them, due to a crucial difference: the OIG Test, unlike the STT, does not make similarity to a human performance the criterion of the test, even though it employs a human performance in setting a criterion for machine intelligence. A man can fail the OIG Test, but it is argued that this is a virtue of a test of intelligence if failure indicates a lack of resourcefulness. It is argued that the OIG Test requires the resourcefulness associated with intelligence and not merely "simulation of human conversational behaviour". The general structure of the OIG Test could even be used with nonverbal versions of imitation games (Sterrett 2000).

Still other writers (Genova (1994), Hayes and Ford (1995), Heil (1998), Dreyfus (1979)) have interpreted Turing to be proposing that the imitation game itself is the test, without specifying how to take into account Turing's statement that the test he proposed using the party version of the imitation game is based upon a criterion of comparative frequency of success in that imitation game, rather than a capacity to succeed at one round of the game.

Predictions and tests

Turing predicted that machines would eventually be able to pass the test. In fact, he estimated that by the year 2000, machines with 109 bits (about 119 MiB) of memory would be able to fool 30% of human judges during a 5-minute test. He also predicted that people would then no longer consider the phrase "thinking machine" contradictory. He further predicted that machine learning would be an important part of building powerful machines, a claim which is considered to be plausible by contemporary researchers in Artificial intelligence.

By extrapolating an exponential growth of technology over several decades, futurist Ray Kurzweil predicted that Turing-capable computers would be manufactured around the year 2020, roughly speaking. See the Moore's Law article and the references therein for discussions of the plausibility of this argument.

As of 2006, no computer has passed the Turing test as such. Simple conversational programs such as ELIZA have fooled people into believing they are talking to another human being, such as in an informal experiment termed AOLiza. However, such "successes" are not the same as a Turing Test. Most obviously, the human party in the conversation has no reason to suspect they are talking to anything other than a human, whereas in a real Turing test the questioner is actively trying to determine the nature of the entity they are chatting with. Documented cases are usually in environments such as Internet Relay Chat where conversation is sometimes stilted and meaningless, and in which no understanding of a conversation is necessary. Additionally, many internet relay chat participants use English as a second or third language, thus making it even more likely that they would assume that an unintelligent comment by the conversational program is simply something they have misunderstood, and are also probably unfamiliar with the technology of "chat bots" and don't recognize the very non-human errors they make. See ELIZA effect.

The Loebner prize is an annual competition to determine the best Turing test competitors. Although they award an annual prize for the computer system that, in the judges' opinions, demonstrates the "most human" conversational behaviour (with A.L.I.C.E. being a recent winner multiple times, and learning AI Jabberwacky in second), they have an additional prize for a system that in their opinion passes a Turing test. This second prize has not yet been awarded. The creators of Jabberwacky have proposed a personal Turing Test: the ability to pass the imitation test while attempting to specifically imitate the human player, with whom the AI will have conversed at length before the test. [1].

Trying to pass the Turing test in its full generality is not, as of 2005, an active focus of much mainstream academic or commercial effort. Current research in AI-related fields is aimed at more modest and specific goals.

There is an ongoing $10,000 bet at the Long Bets Foundation between Mitch Kapor and Ray Kurzweil about the question whether a computer will pass a Turing Test by the year 2029. The bet specifies the Turing Test in some detail.


In Turing's paper, the term "Imitation Game" is used for his proposed test as well as the party game for men and women. The name "Turing test" may have been invented, and was certainly publicized, by Arthur C. Clarke in the science-fiction novel 2001: A Space Odyssey (1968), where it is applied to the computer HAL 9000.

Variations of the Turing test

A modification of the Turing test, where the objective or one or more of the roles have been reversed between computers and humans, is termed a reverse Turing test.

Another variation of the Turing test is described as the Subject matter expert Turing test where a computer's response can not be distinguished from an expert in a given field.

As brain and body scanning techniques improve it may also be possible to replicate the essential data elements of a person to a computer system. The Immortality test variation of the Turing test would determine if a person's essential character is reproduced with enough fidelity to make it impossible to distinguish a reproduction of a person from the original person.

The Minimum Intelligent Signal Test is another variation of Turing's test, but where only binary responses are permitted. It is typically used to gather statistical data against which the performance of artificial intelligence programs may be measured.


  • Alan Turing, "Computing machinery and intelligence". Mind, vol. LIX, no. 236, October 1950, pp. 433-460. Online version: [2] (with copyright permission)
  • B. Jack Copeland, ed., The Essential Turing: The ideas that gave birth to the computer age (2004). ISBN 0198250800
  • H. L. Dreyfus. What Computers Can't Do, Revised Edition, New York: Harper Colophon Books. (1979) ISBN 0060906138
  • J. Genova. 'Turing's Sexual Guessing Game', Social Epistemology, 8(4): 313-326. (1994) ISSN 0269-1728
  • Larry Gonick, The Cartoon Guide to the Computer (1983, originally The Cartoon Guide to Computer Science). ISBN 0062730975.
  • Stevan Harnad (2004) The Annotation Game: On Turing (1950) on Computing, Machinery, and Intelligence, in Epstein, Robert and Peters, Grace, Eds. The Turing Test Sourcebook: Philosophical and Methodological Issues in the Quest for the Thinking Computer. Kluwer.
  • Patrick Hayes and Kenneth Ford. 'Turing Test Considered Harmful', Proceedings of the Fourteenth International Joint Conference on Artificial Intelligence (IJCAI95-1), Montreal, Quebec, Canada. pp. 972- 997. (1995)
  • John Heil. Philosophy of Mind: A Contemporary Introduction, London and New York: Routledge. (1998) ISBN 0415130603
  • Ray Kurzweil, The Age of Intelligent Machines (1990). ISBN 0262111267.
  • James Moor, ed., "The Turing Test: The Elusive Standard of Artificial Intelligence" (2003). ISBN 1402012055
  • Roger Penrose, The Emperor's New Mind (1990). ISBN 0140145346.
  • S. G. Sterrett, "Turing's Two Test of Intelligence" Minds and Machines v.10 n.4 (2000) ISSN 0924-6495 (reprinted in The Turing Test: The Elusive Standard of Artificial Intelligence edited by James H. Moor,Kluwer Academic 2003) ISBN 1402012055
  • S. G. Sterrett "Nested Algorithms and the 'Original Imitation Game Test'," Minds and Machines (2002). ISSN 0924-6495

See also

External links

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