I was asked a question about whether Luke’s Gospel would be considered a primary or a secondary source, historically speaking, for the life of Jesus. Here’s my answer:
In my own work as an ancient historian I find that a “primary” source is not so much the first to record some information but rather a source which carries independent evidentiary weight. In the context of the question, Mark’s Gospel would be a primary source because it’s not depending on any prior source accessible to us. Where Luke simply quotes Mark we might consider him a secondary source because his reliability depends entirely on the reliability of another source we have–Mark.
On the other hand, Luke is a primary source for a lot of stories and sayings that don’t appear in Mark. We might also say that, since Luke had access to sources (oral tradition, interviews, other documents?) dealing with the contents of Mark that he constitutes a primary source, even where he and Mark overlap. If he has other sources of information aside from Mark, and if his use of Mark is selective (as it is), then his decision to include (and thereby endorse) content from Mark could be said to have independent evidentiary weight. Luke seems to know some of Mark’s stories (like the temptation of Jesus or the institution of the Last Supper) from other sources. He also knows a lot of other early tradition which gives him some ability to evaluate the reliability of Mark’s account.
So to the extent that he’s not just parroting Mark, Luke would constitute an independent primary source. What this case shows, though, is that the categories of “primary” and “secondary” sources are blurry at their edges. That’s why the question of how we label someone like Luke is less important than the way we actually handle his evidence.