 Another example: The reductionist program for theoretical concepts (e.g., those of physics) aimed to solve epistemological problems that these concepts pose. The program aimed to reduce theoretical sentences to (classes of) observational sentences. However, the reductions proved difficult, if not impossible, to sustain. Thus arose the suggestion that perhaps the non-observational component of a theory can, without any claim of reduction, be regarded as an implicit definition of theoretical terms. The precise characterization of the non-observational component can vary with the specific epistemological problem at hand. But there is bound to be a violation of one or both of the two criteria, Conservativeness and  Eliminability.[13]