The vast majority of what people “know” about any given subject they only know because someone told them, or they read it in a book or online, or heard someone talk about it online. The point being, we get our knowledge from what philosophers call the testimony of others. This worked out well in our hunter-gatherer past, when a member of a tribe knew everyone else in the tribe. Person A knows that person B is someone capable of or knowledgeable about X, and so person A can trust person B with telling them about X.
In much larger societies, such as in modern nation states, many, if not most, of the people we run into online and in our daily lives will be strangers, or at the very least not people we are intimately familiar with. This means that much of what we are told comes from strangers, who are usually themselves relaying information they were told from yet other strangers. Since we don’t know if these strangers know what they’re talking about, humans have come up with various means of lending their testimony legitimacy and trustworthiness.
