A postulate to aid in model building.

Abstract One of the difficulties frequently encountered in proposing models for biological systems results from the inadequacy of available information to permit a full quantitative description of the model. Further experiments on the system to aid in the derivation of a unique model are often not feasible. If, however, it is possible to perturb the system so that its parameters may take on new values, and the experiments repeated on the perturbed system, further definition of a model may become possible under special assumptions. A postulate is advanced to this end. It is postulated that the perturbation of a system may be expressed in some minimal way. This is equivalent to the introduction of new constraints on the model, which serve to limit the choices possible. Some of the quantitative aspects necessary to lead to a unique model are discussed. A computational procedure that may be applied routinely is also described. An example is given to demonstrate the concepts involved.