And firsthand sources are superior to secondhand sources. Always? And in what way?
Response: Yes. Always because the chance of someone lying without detecting it or ambiguity is greater when the evidence is based on secondary sources than firsthand. If I tell you the shirt I am wearing is black and you cannot see me, you would have to trust me as evidence, but I could be lying. Yet if you saw me yourself, firsthand, wearing a black shirt, then there is no doubt or ambiguity that my shirt is black. Firsthand evidence is more reliable.
That does not mean secondhand evidence is not reliable. It just means that it is not as reliable as firsthand evidence. So the best way to verify whether secondhand evidence is valid is when it is logically deduced from firsthand evidence.
For example, I say the shirt I am wearing is black but you never saw me. However, you have a bunch of reports saying that my shirt is black, that they are eyewitnesses to it, that each of the stories are logical and consistent, and that there is no contrary evidence that my shirt is not black or that they are lying.
In this case, it is very reliable to say my shirt is black based on secondary sources because we can verify firsthand that anytime a story is truthful and it spreads, it has the following attributes:
Logical
Consistent
The majority
And there is no contrary evidence to refute otherwise
For example, Pigs cannot fly. You would agree. Now you can leave your house right and ask people "Do pigs fly", and the results will have the following attributes:
Logical
Consistent
The majority
And there is no contrary evidence to refute otherwise
So since we have firsthand evidence that confirms that these are the attributes of truth, THEN we can logically deduce that any secondhand source that has these attributes is also truthful or the most reliable.
Hope this explains my application of distinguishing truth from falsehood based on firsthand evidence, deductive logic based on firsthand evidence, and how to verify whether a secondary source is reliable.