Who was there? Weigh the sources
A historian’s first question about any source: was its maker there? Two first-hand sources that agree settle a fact. A source written later, or from what someone heard, can’t settle it — or dispute it. (It is the same rule as LensQuest’s, where four people stood in different places.)
One more distinction: a memory written down years later by someone who was there is still first-hand — but memory fades and mixes in what was heard since, so historians weigh it carefully. Here, the later accounts come from people who were not there.