People often make disastrous mistakes by relying on primary witness testimony. But if primary sources are often deeply unreliable, as we've seen, why do we take the trouble to put witnesses in the box in a court room?
I spent a few weeks on a jury in New York City a year ago. The judge warned us not to speculate, or rely on gossip or hearsay, and judge only on the evidence we heard.
However, the judge also always specficially instructs a jury that a major part of their task is to judge the credibility of the witnesses. They have to weigh the evidence they hear, not meekly accept any testimony without question. A jury is supposed to use their common sense experience to decide whether witnesses appear evasive or hesitant or confused.
This is not analysis, in theoretical terms. It is not abstract. It is a matter of common sense and experience. Primary testimony without judgment of credibiliy is useless.
A jury is also expected to be alert for inconsistencies between witnesses. Indeed, the most important aspect of primary sources is not what any one says in isolation, but where anomalies or inconsistencies point to problems in the evidence.
Added to that, the most persuasive and important evidence in most trials is not testimony from particular witnesses or sources. It is hard objective evidence, less subject to distortion. That means sound recordings, video camera evidence, medical reports, or DNA evidence. It means e-mails, internal documents, receipts, tickets or fingerprints. It means records of interviews, transcribed at the time, calendars and measurements of skid marks or damage.
Primary sources are important. But only if you have a layer of judgment to figure out the value of what they say.