The article is devoted to the development of the principles of the linguistic source study, combining both - the linguistic and the source study approaches. A hypothesis, basing on the materials of a wide range of sources (ancient manuscripts and dialectal dictionaries) is proved, in accordance with which linguistic content (that is, the aggregate of the concluded in the source linguistic facts) does not only depend on the character of the source (genre, circumstances of the appearance), but also stipulates (dictates the investigator) the formulation of these or those linguistic problems or definite aspects of the linguistic investigation.