Another necessary condition for knowledge is truth: that's another reason why, if you really do know you are right, you can not probably be wrong. But the slogan 'if I think I know I am right, I am probably wrong' could be be true. And it could be true in at least two different ways: I could in fact be wrong, or I could in fact be right but not, as Alex points out, have good evidence. Lucky guesses aren't knowledge.

It is not difficult to come up with a case where you have knowledge that you would not have had if you did not have a false belief. Suppose you are a medical researcher, doing the experiments to get the medical data is very difficult, and you never would have been able to make the effort if you didn't believe it would enable you to cure the common cold. That belief was alas false; but the data you gathered enabled you to gain new knowledge (though no c0ld cure). This is a case where you would not have had that knowledge were it not for your false belief; but I would not describe this as a case where the justification itself includes a false belief. The new evidence is true, it is just that you would not have found it if it were not for your false belief.

In his Meditations , Descartes thought he found such a circumstance, namely his belief that he himself exists (as a 'thinking thing'). For he couldn't even imagine anything unless he existed. This is a beautiful line, taking us not from our thoughts out to the external world that seems to cause them but from our thoughts in to the mind that must exist in order for us to have them. But pesky philosophers have raised numerous difficulties with Descartes strategy. I'll mention just two. The first is that even if Descartes Cogito ergo Sum maneuver works, it is hard to see how to extend it to show that we know anything apart from the existence of our own minds. The second is that Descartes doesn't really even seem entitled to belief in the existence of his own mind, if that is something with a past or a future, indeed if it is something that is distinct from the thought itself. In the end, the promising 'I think therefore I am' seems to reduce to the less impressive 'There is a...

I agree with you: people don't seem to see only what they are thinking, because negative people sometimes see positive things, and because we are all sometimes surprised by what we see. But to this one might reply that we can be surprised in dreams, even if in dreams we see only what was in some sense already in our minds. Your general question may be how we know that everything we see isn't just a dream. This is a classic philosophical worry, made particularly famous by Descartes in his First Meditation. (If you want to read this wonderful piece, click on 'Early Modern Texts' on the lower right hand corner of this page.) Many philosophers would say, with regret, that we can't prove that what we see isn't all our dream, but nobody believes it in their ordinary lives. And as I've said, the fact that we are sometimes surprised by what we see does suggest that we aren't making the whole thing up. Interestingly, the fact that we are also often not surprised suggests the same thing. What I mean...

Belief does not imply dead certainty. Indeed many philosophers would say that no belief should reach that level, and some philosophers think that beliefs come in degrees, like probabilities. Doubt also seems to come in different levels, and allowing for the possibility of error may correspond to a low level of doubt that is compatible with belief. Of course if the doubt at issue amounts to actually believing that the proposition is false, then that is incompatible with believing it to be true. (Or if beliefs correspond to probabilities, giving the truth of the proposition a probability less than .5 is incompatible with giving a probability greater than .5 .) Since belief is compatible with allowing for the possibility of error, belief is also compatible with an interest in testing. That process might undermine the belief; it also might strengthen it. In religious contexts, however, the term 'belief' may sometimes be used to mean something like 'unshakable faith', in which case there may...

Philosophers who talk about knowledge usually focus on knowing a fact ('knowing-that') or having an ability ('knowing-how'); but 'knowing-a-person' different from either of these types of knowledge. One one level, it is not very difficult: you could truthfully answer 'yes' to the question of whether you know Alexander George if you had say met him and had a few good conversations. And 'Know-who', as in 'Do you know who Alexander George is?' is even easier: you don't have even to have met him. At the other extreme, if 'getting to know Alexander George' required a kind of comprehensive knowledge-that of all the facts relating to him, you will never make it. Fortunately, you can make progress on getting t0 know someone without impossibly having the full story about them, if you can find out more about their personality and about what they think. And I am more optimistic about the possibility of that than you seem to be.

One part of the answer is the 'why-regress'. As you know, whenever someone answers your why-question, you can almost always ask why about the answer, and this seems like it can go on forever. But I don't think that shows that we never explain anything. If I ask why my car won't start, telling me that my battery is dead can be a good answer, even if you don't tell me why my battery is dead (though I might like to know about that too!).

I'm with Richard here: the truth of a proposition cannot be an illusion. In an illusion, the proposition is false. But there might nevertheless be a sense in which truth could be an illusion, if we think that there are representations when in fact there aren't any. This is paradoxical territory, but for example there is a line of thought from Wittgenstein, articulated in Saul Kripke's Wittgenstein on Rules and Private Language , according to which our thoughts do not have determinate content. No determinate content, no determinate proposition, so no truth. If that were the situation, but we thought that there was truth here, we might (truly?) say that truth is an illusion.

It's hard to generalise about this. For example, is the certain thing a new kind of thing or not.? If not, withholding judgment may be the right answer. For example, I have no reason whatsoever to believe that the house next to mine has a red ball in it, so I probably should simply withold judgment: maybe it does and maybe it doesn't. On the other hand, if I have no reason whatsoever to believe that in the house next to mine there is a kind of animal quite different from any known animal, I probably should deny that it exists. Why should we deny the existence of radically new kinds of things when we have no evidence for their existence? Part of the answer may be that our principles of reasoning incorporate various kinds of preferences for simplicity. In the case of things, this is known as Occam's razor: don't multiply entities beyond necessity. It is not obvious why, taken as a principle of denial rather than a principle of withholding judgment, this is a good principle, but many would say...

There seems to be nothing incoherent in the idea that there might be certain people who are much more reliable about the future than the rest of us, though neither we nor they can account for the source of this extraordinary talent. I don't think it is likely that there are any such people about; but if there are this might have an entirely natural explanation (even if we can't quite figure it out). Perhaps they are much more sensitive to sensory evidence and its significance than the rest of us.