Towards a general theory of computability

There are nowadays two kinds of computers: digital computers and analogue computers. The principal distinction is this. The questions and answers of a digital computer are presented in symbolic form, perhaps printed on a tape or on cards. An analogue computer, on the other hand, relies on the dependence of one physical magnitude on another. For example, the question (or the input) may be an electrical voltage, and the answer (or output) another voltage related to the first in a manner determined by the structure of the machine.