Исторические источники
Primary sources are distinguished from secondary sources, which cite, comment on, or build upon primary sources. Generally, accounts written after the fact with the benefit (and possible distortions) of hindsight are secondary. A secondary source may also be a primary source depending on how it is used. For example, a memoir would be considered a primary source in research concerning its author or about their friends characterized within it, but the same memoir would be a secondary source if it were used to examine the culture in which its author lived. "Primary" and "secondary" should be understood as relative terms, with sources categorized according to specific historical contexts and what is being studied.