<*biography, history of philosophy*> English mathematician
(1912-1954). Extrapolating from the mathematical discoveries of
G–del, Turing proposed in Computing Machinery and Intelligence
(1950) a specific description of just what an idealized machine
could, in principle, compute. In addition to its practical
importance for the development of digital computing equipment,
Turing's theory provides support for a functionalist account
of the mind by proposing the practical test of whether or not we
would attribute intelligence to a system whose performance is
indistinguishible from that of a human agent.
Recommended Reading: Alan Mathison Turing, Mathematical Logic
(Elsevier, 2001); Andrew Hodges, Alan Turing: The Enigma (Walker,
2000); Andrew Hodges, Turing (Routledge, 1999); and Machines and
Thought: The Legacy of Alan Turing, ed. by Peter Millican and Andy
Clark (Oxford, 1999).

[A Dictionary of Philosophical Terms and Names]

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<*computability*> A hypothetical machine defined in 1935-6 by
Alan Turing and used for computability theory proofs. It
consists of an infinitely long "tape" with symbols (chosen
from some finite set) written at regular intervals. A
pointer marks the current position and the machine is in one
of a finite set of "internal states". At each step the
machine reads the symbol at the current position on the tape.
For each combination of current state and symbol read, a
program specifies the new state and either a symbol to write
to the tape or a direction to move the pointer (left or right)
or to halt.

In an alternative scheme, the machine writes a symbol to the tape *and* moves at each step. This can be encoded as a write state followed by a move state for the write-or-move machine. If the write-and-move machine is also given a distance to move then it can emulate an write-or-move program by using states with a distance of zero. A further variation is whether halting is an action like writing or moving or whether it is a special state.

[What was Turing's original definition?]

Without loss of generality, the symbol set can be limited to just "0" and "1" and the machine can be restricted to start on the leftmost 1 of the leftmost string of 1s with strings of 1s being separated by a single 0. The tape may be infinite in one direction only, with the understanding that the machine will halt if it tries to move off the other end.

All computer instruction sets, high level languages and computer architectures, including parallel processors, can be shown to be equivalent to a Turing Machine and thus equivalent to each other in the sense that any problem that one can solve, any other can solve given sufficient time and memory.

Turing generalised the idea of the Turing Machine to a "Universal Turing Machine" which was programmed to read instructions, as well as data, off the tape, thus giving rise to the idea of a general-purpose programmable computing device. This idea still exists in modern computer design with low level microcode which directs the reading and decoding

of higher level machine code instructions.

A busy beaver is one kind of Turing Machine program.

Dr. Hava Siegelmann of Technion reported in Science of 28 Apr 1995 that she has found a mathematically rigorous class of machines, based on ideas from chaos theory and neural networks, that are more powerful than Turing Machines. Sir Roger Penrose of Oxford University has argued that the brain can compute things that a Turing Machine cannot, which would mean that it would be impossible to create artificial intelligence. Dr. Siegelmann's work suggests that this is true only for conventional computers and may not cover neural networks.

See also Turing tar-pit, finite state machine.

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<*artificial intelligence*> A criterion proposed by Alan
Turing in 1950 for deciding whether a computer is
intelligent. Turing called it "the Imitation Game" and
offered it as a replacement for the question, "Can machines
think?"

A human holds a written conversation on any topic with an unseen correspondent (nowadays it might be by electronic mail or chat). If the human believes he is talking to another human when he is really talking to a computer then the computer has passed the Turing test and is deemed to be intelligent.

Turing predicted that within 50 years (by the year 2000) technological progress would produce computing machines with a capacity of 10**9 bits, and that with such machinery, a computer program would be able to fool the average questioner for 5 minutes about 70% of the time.

The Loebner Prize is a competition to find a computer program which can pass an unrestricted Turing test.

*Julia* is a
program that attempts to pass the Turing test.

See also AI-complete.

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