On the theory of models and the modelling of natural phenomena.J.CastiauthorIt is argued that a modelling relation in an encoding of a set of physical observables, characterising a natural system N, into an appropriate formal mathematical structure. The theorems of the formal structure are then decoded into true statements about N. Various system-theoretic concepts such as bifurcation, similarity, system analogy, reductionism, and complexity are then seen to be naturally expressed.1984-09GowerBook Section