text: When multiple historical stages of a single language exist, the oldest attested stage is normally termed "Old X" (e.g. Old English and Old Japanese). In other cases, such as Old Irish and Old Norse, the term refers to the language of the oldest known significant texts. Each of these languages has an older stage (Primitive Irish and Proto-Norse respectively) that is attested only fragmentarily.
  Accuracy
  There are no objective criteria for the evaluation of different reconstruction systems yielding different proto-languages. Many researchers concerned with linguistic reconstruction agree that the traditional comparative method is an "intuitive undertaking."
  The bias of the researchers regarding the accumulated implicit knowledge can also lead to erroneous assumptions and excessive generalization.  offers several examples in where such general assumptions concerning "the nature of language" hindered research in historical linguistics. Linguists make personal judgements on how they consider "natural" for a language to change, and "[as] a result, our reconstructions tend to have a strong bias toward the average language type known to the investigator."
  Such an investigator finds themselves blinkered by their own linguistic frame of reference.
  The advent of the wave model raised new issues in the domain of linguistic reconstruction, causing the reevaluation of old reconstruction systems and depriving the proto-language of its "uniform character." This is evident in Karl Brugmann's skepticism that the reconstruction systems could ever reflect a linguistic reality. Ferdinand de Saussure would even express a more certain opinion, completely rejecting a positive specification of the sound values of reconstruction systems.