The Gospels may not have been written as objective, disinterested accounts of what really happened in the life of Jesus, but they clearly do contain historical information. The trick is figuring out what is historical and what is legendary. — Bart Ehrman: “The Historians Wish List”
They “clearly do contain historical information”? Clearly? How do we know?
There are some details that can be corroborated by independent sources, such as the existence of Pharisees, Roman authority over Judea, cultic practices around the Jerusalem temple, and so forth. But without those independent witnesses we would have no way of knowing that even those details were “clearly historical information”.
Bart Ehrman does point out the existence of “external” sources in Jesus: Apocalyptic Prophet of the New Millennium — e.g. Pliny, Tacitus. Yes, their writings are certainly “external” to the gospels but to what extent they are “independent” or even authentic is another question that the historian is required to assess prior to his/her use of them. Ehrman calls them “external checks” on the gospels, but they can only be “checks” (p. 53) if they can be established to be independent. If they derive from a time much later than the events narrated in the gospels then questions inevitably arise about their independence of knowledge of the canonical gospel story. (In the case of Pliny we have serious questions about the authenticity of the key letter, not to mention the letter’s failure to even mention “Jesus” per se.)
(Note: we have seen in case studies of Demonax and Gyges on this blog that an external source can be late and still be reasonably argued to contain independent information and it can be contemporary and found to be false. But arguments need to be provided; the simple fact of lateness or contemporaneity alone does not automatically rule out or in the value of evidence. Comparable arguments would need to be supplied for the claims found in Tacitus for Tacitus to be considered an “external check” on the gospel accounts.)
It is one thing to know that documents contain or hide historical information in or behind their narratives and from that foundation proceed to see what we might consider historical. But it is quite another exercise to come to that prior certainty that the documents “clearly do contain historical information” that can be extracted somehow.
If we start applying methods to extract information of a certain kind before first establishing that the source is a genuine repository of that information, then we are putting the cart before the horse. Our exercise becomes a circular process. We will declare our extracted information “historical” (or “probably historical”) and possibly use that result to go back and argue that our documents “clearly do contain historical information.”