It might be urged that when playing the “imitation game” the best strat- egy for the machine may possibly be something other than imitation of the behavior of a man. This may be, but I think it is unlikely that there is any great effect of this kind. In any case there is no intention to investigate here the theory of the game, and it will be assumed that the best strategy is to try to provide answers that would naturally be given by a man.
The question which we put in § 1 will not be quite definite until we have spec- ified what we mean by the word “machine.” It is natural that we should wish to permit every kind of engineering technique to be used in our machines. We also wish to allow the possibility that an engineer or team of engineers may construct a machine which works, but whose manner of operation cannot be satisfactorily described by its constructors because they have applied a method which is largely experimental. Finally, we wish to exclude from the machines men born in the usual manner. It is difficult to frame the definitions so as to satisfy these three conditions. One might for instance insist that the team of engineers should be all of one sex, but this would not really be satis- factory, for it is probably possible to rear a complete individual from a single cell of the skin (say) of a man. To do so would be a feat of biological technique deserving of the very highest praise, but we would not be inclined to regard it as a case of “constructing a thinking machine.” This prompts us to abandon the requirement that every kind of technique should be permitted. We are the more ready to do so in view of the fact that the present interest in “thinking machines” has been aroused by a particular kind of machine, usually called an “electronic computer” or “digital computer.” Following this suggestion we only permit digital computers to take part in our game.
This restriction appears at first sight to be a very drastic one. I shall at- tempt to show that it is not so in reality. . . . The digital computers considered [here] are classified among the “discrete state machines.” These are the ma- chines which move by sudden jumps or clicks from one quite definite state to another. . . . [A] special property of digital computers [is] that they can mimic any discrete state machine. [This property] is described by saying that they are universal machines. The existence of machines with this property has theVimportant consequence that, considerations of speed apart, it is unnecessary to design various new machines to do various computing processes. They can all be done with one digital computer, suitably programmed for each case. It will be seen that as a consequence of this all digital computers are in a sense equivalent. . . .It was suggested tentatively that the question, “Can machines think?” should be replaced by “Are there imaginable digital computers which would do well in the imitation game?” If we wish we can make this superfi- cially more general and ask “Are there discrete state machines which would do well?” But in view of the universality property we see that either of these questions is equivalent to this, “Let us fix our attention on one particular digital computer C. Is it true that by modifying this computer to have an ad- equate storage, suitably increasing its speed of action, and providing it with an appropriate program, C can be made to play satisfactorily the part of A in the imitation game, the part of B being taken by a man?” . . .