It is quite clear the way in which we can, and perhaps should, consider mathematical logic as a branch of mathematics, in fact a part of applied mathematics. The main technique used in applied mathematics is that of studying a phenomenon by building a mathematical model of it. Such a model is an idealization which approximates the phenomenon by concentrating on a few relevant features and ignoring what seems to be less significant. If the model turns out to have high predictive and explanatory value, at least in some useful situations, it is subjected to extensive theoretical development and becomes a ‘theory’ (i.e. probability theory, heat theory, etc.).