I can see why you suggest that "I am therefore I think" is a better way of putting things: existence is necessary for thinking in a way that thinking is not necessary for existing. Indeed existing is necessary for a thing to have any properties whatever, whereas there are things that exist but do not think. But Descartes was here not interested in the order of reality ; instead, as Sean points out, he was interested in the order of knowledge . And he comes to know that he exists by means of his awareness of his thinking, not vice versa .

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.

In spite of the enormous interest in astrology over thousands of years and by some very bright people, there seems to be no good reason whatever to believe that the position of the planets when you are born affects your future.

Lexicography and conceptual analysis must anyway be different, since a good dictionary has lots of good definitions, while philosophers are hard pressed to give a correct conceptual analysis of any of the concepts that centrally concern them. One reason for this may be that conceptual analysis tries to go deeper than lexicography, to give the underlying contours of a concept. Take the concept of knowledge. My dictionary defines knowledge as 'the facts, feelings or experiences known by a person or group of people'. Not very deep, because it uses 'known' in the definition. Philosophers try to avoid that sort of circle. The traditional philosophical definition of knowledge is 'justified true belief', which probably avoids a circle; but it is also wrong. Like pretty much every philosophical analysis, it is both too narrow and too broad. Thus you can know without having a justification, say if you just see something, and you can have a justified true belief without knowing, because it may still be just...

I think you can. You clearly could dream you were walking naked through your local supermarket while you are walking naked through your local supermarket, and I think the same could be said for imagining, though you would have to be an exceptionally absent-minded philosopher for this actually to happen. To take a less extreme example, you might imagine that you were traveling through Luxembourg at a time when, although you didn't realise it (because it was dark and Luxembourg is such a small country that it is easy to miss), you actually were driving through Luxembourg.

This is a question that has bugged philosophers since at least Plato. Suppose you have a clever, rational jerk, that is, a person who just doesn't want to do the right thing. Is there any argument we can give him that would force him to be moral if he remains rational? I have not seen such an argument, so I'm inclined to suppose that moral motivation depends on the existence of some desire to be decent. But perhaps one could argue that a person without a shred of moral motivation or sympathy for others would be so different from you or me that we would have trouble seeing such a person as a rational agent rather than as a monster.

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.

Being hypocritical might mean that I argue for a standard of behavior that I do not myself meet. Thus I might argue that all parents should send their children to public (state) schools, even though I send my children to a private school. My behavior does not undermine whatever force my argument may have. In that sense, it is not wrong. But there is a sense in which I must be getting something wrong: either the claim I am arguing for or my behavior. Still, I am not necessarily misrepresenting myself. If on the other hand, what you mean by 'being hypocritical' is that I misrepresent my own position, say by saying that parents ought to send their children to public school when I don't believe that, then we haves the wrong of dishonesty.

I'm no Plato expert, but I think the main issue here is not how we judge or know what is just, but rather what justice is. If it is not just to do good to an evil friend, this shows that justice isn't the same as doing good to a friend. And it shows this no matter how easy it is for us to identify our friends and to judge their worth.

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...