Mathematical models in science usually have free parameters that should ideally be estimated from empirical data. Sometimes, however, model parameters cannot be estimated because pertinent empirical data are simply not available. A colleague and I are looking for examples of such models in the scientific literature (economics, biology, medicine, physics etc.), and we would be grateful for any pointers. Specifically, these would be models that had some heuristic value (e.g., through generating predictions for various plausible parameter combinations), despite the fact that the parameters could not directly be estimated.