Mind: A Brief Introduction (John R. Searle) / quote 66 of 138
The Turing machine as an abstract concept
A Turing machine is a device that carries out calculations using only two types of symbols. These are usually thought of as zeros and ones, but any symbols will do. The idea of the Turing machine was invented by Alan Turing, the great British logician and mathematician. The striking feature of the Turing machine is its simplicity: it has an endless tape on which the symbols are written. It has a head that reads symbols on the tape. The Turing machine head will move to the left or to the right, it can erase a zero, it can print a one, it can erase a one, it can print a zero. It does all of these things in accordance with a program, which consists of a set of rules. The rules always have the same form; under condition C, perform act A: C A. For example, a rule might be of the form, if you are scanning a zero, replace it with a one and move one square to the left. A Turing machine is not a machine in the ordinary sense. You cannot go into a store and buy a Turing machine. It is an abstract mathematical concept.