How can we rely on testimony?

Much of what we claim to know rests not on our own direct experience, but on the word of others. You seem to know that Mount Everest is the tallest mountain on Earth, but you haven't measured its height yourself. You also seem to know that electrons exist, but you have never seen one with your own eyes. Every day, our beliefs are built on testimony—from the mundane ("the bus will arrive at 7:15") to the profound ("vaccines save lives" or "black holes bend spacetime").
Yet testimony raises deep philosophical questions. How can we justifiably trust the reports of others? Can one person's word ever serve as evidence? Philosophers since David Hume (1711-1776) and Thomas Reid (1710-1796) have debated whether testimony is a basic (i.e. fundamental) source of knowledge, on par with sense-perception and memory, or whether it is only credible when backed by other sources that we already trust.
Science is often presented as the antidote: its claims are, in principle, reproducible by anyone. But in practice, no single individual can reproduce the high-energy particle experiments, the measurement of the cosmic microwave background, or even the determination of Everest's height. Instead, we rely on a vast network of specialists, institutions, and instruments, accepting testimonies that we could never personally verify. And when we fact-check a surprising claim, we do not set up an experiment ourselves: we rather seek out other voices we judge more reliable.
At this meetup, we'll explore the philosophy of testimony. What makes us treat some voices as trustworthy and others as doubtful? And what does all this mean for how we navigate a world where nearly all of our knowledge is secondhand?
Discussion Questions:

• If a close friend swore they had seen a ghost, would you believe them? What would it take for you to change your mind?
• Do we have good reason to trust scientists more than journalists, or journalists more than our own family and friends?
• When conspiracy theorists urge us to "do your own research", are they rejecting testimony or just replacing one set of authorities with another?
• Could you still claim to actually know anything about the world if you resolved never to trust anyone else's word?
• Should an automatic trust in institutions be seen as rational confidence—or as a dangerous form of intellectual laziness?