In linguistic cases of doubt a differentiation has to be made between the pragmatic reflections of language by laypersons (SZ 1) and the common 'extra-communicatice' reflections by linguists (SZ 2). These structural inhomogeneities of languages are based upon (e.g. ortho-graphic) variants, 'openings', so called 'language fluctuations' or some other irregular forms that have developed historically (SZ 2 = 'objective imperfectibility' of the linguistic system). On the other hand, the competence theoretical notion of 'linguistic doubt' (of the type SZ 1) requires that speakers become aware or are aware of their lacking, constricted, or insecure knowledge in their language usage (= SZ 1: 'subjective imperfectibility'). From a point of linguistic development, both linguistic cases of doubt can be seen as indicators for potential 'mutants' in the inheritance of linguistic forms, structures and functions. Hence, 'imperfectibility' (both as a quality of a system and as a cognitive substrate of it) is an important condition for future linguistic change.