Nestorchronik
Danilevskiy shows that the oldest historical source, the "Primary Chronicle", is inconsistent in what it refers to as the "Rus'": in adjacent passages, the Rus' are grouped with Varangians, with the Slavs, and also set apart from the Slavs and Varangians. Danilevskiy suggests that the "Rus'" were originally not a nation but a social class, which can explain the irregularities in the "Primary Chronicle" and the lack of early non-Slavic sources.