structural parameter in modelling

Acemoglu 2010:22 (Theory, General equilibrium, and political economy in development economics. J Ec Perspectives. v24 n3 :17-32) "What we take as a structural parameter for one theory will naturally become an endogeneous object in another. So a particular model can serve us well as an abstraction for a series of counterfactual experiments, but there will exist other experiments for which it will be much less informative. This is almost by necessity: a precondition for external validity is that key factors relevant for the outcome of the counterfactual should be included in the model, and models as abstractions have to exclude several relevant factors, so no single model can include all of the relevant factors for all possible counterfactual exercises."