i) "Secondhand information" often has a pejorative connotation, by way of invidious contrast to firsthand information. It can be a synonym for rumor, scuttlebutt, or unconfirmed reportage. Something heard through the grapevine. "Hearsay" has the same pejorative connotations.
This is relevant to debates about the historical Jesus. For instance, since Luke's Gospel is secondhand information, does that make it inferior?
ii) To begin with, we need to distinguish between oral tradition and oral history. Oral tradition connotes a saying or story that was passed down by word of mouth from person to person until it was finally committed to writing. There are many links in the chain of transmission, with many opportunities for the original saying or story to be modified in the process of tradition.
By contrast, oral history has one source. Straight from the mouth of the eyewitness.
iii) Literally, secondhand information means information at one remove from the original source, but in popular usage it allows for however many intervening steps. Suppose, though, we use the word in the literal sense. Let's consider the potential reach of literal secondhand information. Consider the potential reach of living memory.
Many people have firsthand information about their grandparents. They personally know one or more of their four grandparents. By the same token, many of their grandparents had firsthand information about their own grandparents. Your grandparents can share what they directly knew about their grandparents with you. That means you can have secondhand knowledge of your great-great grandparents. There's just one link between you and your great-great grandparents. Even though that's five generations deep, that's still just secondhand knowledge. It's not fourthhand or fifthhand knowledge . You can have direct knowledge of your grandparents. Skipping a generation (your parents) doesn't make that secondhand information. You don't have to get your information about your grandparents from your parents, if you personally know your grandparents. Even though we're adding generations, we're not adding intervening links between you and the original source. Although we've now gone back five generations (child>parent>grandparent>great-grandparent>great-great-grandparent), it isn't four or five steps removed from the original. It's still only one step removed from the original source of information.
In addition, many people personally know their great-grandparents. And some great-grandparents knew their own great-grandparents. That goes back seven generations. We've added your firsthand knowledge of your great-grandparents and their firsthand-knowledge of their great-grandparents. That means some people can have secondhand information about their great-great-great-great grandparents. But it's not sixthhand or seventhhand information. It's still just secondhand information. If you have direct knowledge of your great-grandparents, and they have direct knowledge of their great-grandparents, then your source of information about your great-great-great grandparent remains just one step removed from the original source. They know what their great-grandparents said and did direct from their own mouth, and you know what your great-grandparents said and did direct from their own mouth. They can share their firsthand knowledge of their great-grandparents with you, while you have can have firsthand knowledge of your own great-grandparents. You have firsthand knowledge four generations deep (about yourself, your parents, your grandparents, and your great-grandparents), and your grandparents have firsthand knowledge four generations deep.
Indeed, some people even know their great-great grandparents, and some of them knew their great-great grandparents. Yet that's still just secondhand knowledge, in the literal sense that there's only one link between your living memory and their living memory.
Although that's statically rare, given billions of people, there's still a large number of people for whom that's true.
iv) Now let's switch to another aspect of secondhand information. Here I'm using the term in a looser sense, but not a pejorative sense.
It's quite possible for secondhand information to be more reliable than firsthand information. Compare a biography to an autobiography. Oftentimes, one function (sometimes the primary purpose!) of autobiographies is to define their reputation for posterity. It's not just a record of what they remember, but how they wish to be remembered. The result may be misleading to one degree or another depending on how many liberties they take with the truth.
Firsthand accounts can be very partisan. Consider political memoirs.
By contrast, a biography may be more candid because it isn't the biographer's reputation that's on the line. So he doesn't have the same personal agenda. Same thing with a historian.
v) Likewise, some people have biased memories. Even though these are firsthand recollections, what they recollect may be less accurate than a secondhand source.
vi) On a related note, as a kid I saw lots of films and TV shows back in the 60s. I have partial memories of what I saw. Sometimes, out of curiosity, I will Google them to fill in the gaps in my memory. My firsthand knowledge is sufficiently accurate to pick the right search terms. But when I pull up secondhand information, it freshens my recollection of forgotten or occasionally misremembered details. In that respect, the secondhand information can be more accurate than my firsthand knowledge of movies or episodes I saw just once decades ago.
vii) In addition, an autobiography narrates events from one source and one perspective: only what the autobiographer saw, heard, and did. By contrast, a biographer or historian may have multiple sources of information. So his treatment may be more complete or evenhanded.
viii) Finally, an autobiographer will be emotionally invested in his own life-experience. By contrast, a biography or historian can bring more critical detachment to bear precisely because it didn't happen to him. He doesn't have those emotionally charged memories. He doesn't personally identify with events in the same way a participant does. So he can sift the evidence more dispassionately.
I'm not saying historians and biographers can't be biased. And I'm not saying autobiographers can't be self-critical. I'm just examining knee-jerk assumptions.
Suppose we bracket inspiration for the sake of argument. And suppose we grant the traditional authorship of Luke's Gospel and John's Gospel. In principle, Luke's "secondhand" Gospel could be more reliable than John's "firsthand" Gospel.
Now, I don't think that's actually the case. But even when we factor in verbal, plenary, organic inspiration, a firsthand account and a secondhand account can still complement each other.