Knowledge

If I am certain that p but another person is certain that ~p, does the bare fact of his certainty give me reason to doubt my own?

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This is actually a fascinating question. I think the answer must surely depend on the circumstances. Why, precisely, do you feel so certain that p is true? There are a couple of possible answers to this. It could be that p just strikes you as so obvious that it has never really occurred to you to question it at all. Simply in order for us to be able to live our lives and get around in the world, we need to believe innumerable things, far too many for us ever to conduct a thorough investigation into the evidence for each and every one. It's perfectly normal for us to feel no doubt whatsoever about many things, for no other reason than that we've heard them repeated over and over without ever encountering a dissenting voice. And this shouldn't be regarded as an epistemic failing: but, unfortunately, such beliefs do sometimes turn out to be false. A case which is often presented to illustrate this fact is that, back in the Middle Ages, the predominant opinion was that the Earth was flat; a belief which, no matter how much certainty people might have felt in its truth at that time, was nevertheless false. A case which I rather prefer to use as an example is that, here and now in the twenty first century, the predominant opinion is that, back in the Middle Ages, the predominant opinion was that the Earth was flat. As it happens, this is a complete myth, a nineteenth century fabrication. A modicum of research into the history of ideas will clearly demonstrate that the predominant opinion across mankind, almost as far back as human history reaches, was consistently that the Earth was round. And yet most people nowadays don't realise this, because they've had the myth reinforced by hearsay so often that they've never thought to question it.

So suppose, hypothetically, that you feel completely certain about a belief like this, without ever really reflecting on whether you have adequate grounds for such a high level of confidence. If somebody else, someone who has actually scrutinised the evidence that you never bothered to look into yourself, then informs you that they are certain of the contrary, then I think it would be appropriate for you to withdraw your claim to certainty. Or at least to withhold it until such time as you have properly assessed this person's evidence and arguments. Perhaps further research will show that their reasons don't hold water, and that your initial conviction was the correct one after all: but it's surely worth taking the trouble to find out. If an expert claims to be certain of something, it is certainly no proof that it is true, but it does seem to constitute prima facie evidence that there might be something here worth taking into account before you form a firm judgment in the matter.

Conversely, though, perhaps you are the expert. Perhaps you have fully explored all of the relevant considerations, and come to a conclusion on that basis. If, then, somebody comes to you claiming certainty in the contrary, but you can diagnose the reason why they believe as they do and can recognise that it is not a good reason -- if, for instance, their belief is blatantly grounded in unexamined hearsay -- then I can't see any real need for you to allow their certainty to undermine your own.

Then again, if you are both erudite, reflective people, who have both taken the time to scrutinise the evidence and arguments, and yet have still come to embrace contrary viewpoints, then perhaps you should both avoid claiming certainty. Fundamental disagreements do regularly crop up even within communities of experts, on pretty much any topic you could care to mention. If you happen to be involved in such a dispute, you might feel enormously confident in your own position, and you might feel that your opponent must surely have missed something. But, recognising and respecting your opponent's acumen and expertise, you should still reserve at least a small corner of your mind for the notion that perhaps you are the one who has missed something. None of us are omniscient and, although some opposing viewpoints and some opponents can be straightforwardly dismissed as not worthy of our attention, others cannot be.

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