I feel that it is worth while to give a brief description of Church’s scheme not only because it emphasizes that computability is a mathematical idea, independent of any particular concept of computing machine, but also because it illustrates the power of abstract ideas in mathematics. The reader who is not readily conversant with mathematical ideas, nor intrigued by such things for their own sake, may, at this stage, prefer to move on to the next chapter – and there would not be significant loss in the
