Models are the basis for all prediction of system behavior, and hence form a crucial element of engineering design. A key concern is the validity of such models. This paper discusses the notion of model validity and the limits of what one can say about the validity of a specific model. It is shown that predictive models, such as those used in engineering design, cannot be validated objectively. That is, the validation of a predictive model can be accomplished only in the context of a specific decision, and only in the context of subjective input from the decision maker, including preferences.