In addition to boasting about witnessing part or much of an event,1 historians were especially quick to boast if they essentially saw it all from the beginning.
This is relevant because, if having “seen it all” was cause for boasting from historians, then witnesses who “saw it all” were especially valued by them. We might expect from this value that historians would’ve then exerted some effort to acquire such testimony directly themselves or through direct witnesses.
The phrase (e.g. “eyewitnesses from the beginning”) functions as a technical term frequently employed to donate historiographic intentions.
This page analyzes one kind of evidence…
This is relevant because the reason this phrase was associated with historiography is that it was considered important, as far as possible, to use witnesses who “saw the whole thing.”
1st-2nd century historiographers did not strive to ground their material in witness approval directly nor nearly directly. See here.