The second one:
Previously, I said:
This is not pure intellectual wankery about semantics: people make serious mistakes in their epistemology that can sound reasonable with an intuitive notion of evidence, and are shown to be deeply flawed if one takes a more rigorous approach.
Time to pay up on that.
Not long ago someone in the sceptic community made a remark to the effect of “All claims, extraordinary or not, require evidence. And by definition, a claim is not evidence in itself”*. I would not be surprised if a substantial number of the audience thought the notion to be perfectly reasonable, as opposed to entirely absurd.
“What could possibly be absurd about that?”, you ask. “You're a sceptic, and surely, the notion that you should have evidence for your beliefs is a fundamental tenet of scepticism” Indeed it is! That's not what I object to. “Then you believe a claim counts as evidence for itself? That completely nullifies the idea you just agreed to. What's the point of asking for evidence for beliefs if you're just going to take the belief as evidence?” Well, that is why I wrote that whole other thing on the nature of evidence.
If you read my previous post, you might notice that I was careful to speak of evidence for 'hypotheses', rather than 'claims'. Why? Well, other than the fact that it doesn't sound quite as pretentious as 'hypothesis', 'claim' has the inherent implication that it's something someone is saying. A hypothesis exists independently of whether someone proposes it, a claim does not. Which means that, when you encounter a claim, you have already made an observation
. Specifically, you have observed that someone is making that claim.
“Big deal”, you say. “People say all sorts of weird shit, doesn't make them true”. But the point is not that evidence makes beliefs true, remember? It's that it makes them more probable. “And? Why does someone saying something make it more probable?” I'm glad you asked, imaginary person whose dialogue I'm writing to move this piece along.
In the general case
, there is no guarantee that someone saying something should cause you to assign it higher probability, that much is true. For example, if you suspect someone is secretly trying to kill you and they are giving you advice about which types of berries are poisonous, you'd probably do better by doing the opposite of what they say (unless they expect you to do that).
But that is not every case, and there are plenty of situations where an assertion being made should make you judge it more probable. If you think the person knows the answer, and you don't have any strong expectation for them to lie, for example. If they are a respected authority in a field relevant to the claim. If it's a question you never considered before, but your interlocutor has done so at length, and you don't think them an idiot.
People's beliefs, people's assertions, they are facts about the world. They are correlated with aspects of the world**, much in the same way as the reading of a thermometer is correlated with temperature. To specifically exclude a particular kind of fact about the world from qualifying as evidence is completely arbitrary; but if you think in terms of “claims” and “justifying beliefs”, this might not be apparent. If you think in terms of updating the probability of a hypothesis, it is somewhat more so.
“Which is nice and all, but isn't there a problem here? You argue that your notion of evidence is correct, because it gives correct answers here where intuitive notions can fail. And you argue intuitive notions fail here, based on your previous definition of evidence! Circular, innit?”
Not quite. The probabilistic view of evidence provides a good illustration of the fallacious nature of “claims cannot be evidence”, but it is not at all required to show that fallacy. Consider: I meet a guy and he tells me his name is Paul. If I refuse to accept this claim as evidence for itself and demand he show some ID, not only am I being rather rude, but I'm also being stupid. It is a fact that, very nearly every time, if you meet someone and they tell you their name, they are telling the truth. I have anecdotal evidence that corroborates this; every time someone tells me their name and I later find out independently, from looking at ID or school records or something, the names matched. I don't think that's a coincidence.
“But scepticism is not about navigating basic social interactions! It's about other sorts of claims, like UFOs and alternative medicine and stuff.”
Reality does not draw a sharp boundary between “stuff sceptics usually apply scepticism to” and “all that other stuff”. If you have a theory of how to obtain reliable knowledge
, there's no particular reason it should work in a fundamentally different manner for homoeopathy and for Paul's name. The techniques you use might be different (I am certainly not suggesting that you take homoeopaths at their word!), but the deep-level foundations should be the same, and account for why you treat both cases differently. In this case, it's obvious: Paul knows his own name, much like most people know theirs, and has no reason to lie to me. Whereas homoeopaths cannot say how they know homeopathy works, and even if they know it doesn't, they have an economic incentive to sell it.
If you epistemology doesn't work properly in the simple case of finding out people's names, that hints at the idea that it's not quite right. Certainly sceptics usually deal in cases where the claim itself is very weak evidence, or maybe no evidence at all, but that's not a fundamental property of evidence, rather a result of a selection effect: if it was a simple enough question that we can just take people's word for it, we probably don't need the systematic approach of scepticism. Evidence is still the same thing, and to treat it differently can lead to error.
* The specific person, as well as the claim they were responding to, are mostly irrelevant to the point I wish to make.
** Not in the general case, again, but it is so in specific cases. My belief that the sky is blue is correlated with the colour of the sky, because I formed it by observing the sky. Consider that to say that people's beliefs are uncorrelated with the world, is to say that everything you
believe, you believe it for no good reason and could just as well be false. And if you think you are the only one whose beliefs are correlated with reality, well... what evidence do you have for that?