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Computing Machinery and Intelligence

Turing, 1950, Computing Machinery and Intelligence (Mind)

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5 cards in “5 Universality of digital computers” · whole deck

5 Universality of digital computers

  1. What is a discrete-state machine?
    A machine that moves by sudden jumps from one definite state to another, the states being different enough that confusion between them can be ignored. Strictly, nothing physical is like this, but many machines can usefully be thought of so.
    q1q2q3
    input i0 (lever off)q2q3q1
    input i1 (lever on)q1q2q3
    outputo0o0o1 (lamp)
    Turing’s example: a wheel clicking round through three positions, stopped by a lever, with a lamp lit in one position.
  2. How do discrete-state machines differ from Laplace’s predictable universe?
    In the physical universe tiny errors in initial conditions can have overwhelming later effects (one displaced electron may decide whether a man is killed by an avalanche a year later). In a discrete-state machine this doesn’t happen: reasonably accurate knowledge of the state yields reasonably accurate prediction any number of steps later.
  3. What is a machine’s “storage capacity” in Turing’s sense?
    The base-2 logarithm of its number of states, so capacities of combined machines add. The three-state wheel has about 1.6.
  4. What does it mean that digital computers are universal machines?
    Given the table of any discrete-state machine, a digital computer with enough storage and speed can be programmed to mimic it, so the interrogator couldn’t tell them apart. Hence, speed aside, all digital computers are in a sense equivalent, and one machine suitably programmed replaces designing new ones.
  5. Thanks to universality, what does the imitation-game question reduce to?
    Whether one particular digital computer, given adequate storage, greater speed and an appropriate programme, can be made to play the part of A satisfactorily, with a man as B.

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