My first reaction was that you must be rather different from me, because I sure don't hear my voice when I'm not speaking. But on inspection I find that I can of course imagine myself speaking (actually I find it works better for singing), which is at least a lot like hearing my voice in my head without speaking. But this is no more mysterious than the fact that I can imagine other sounds without actually hearing them, or that I can remember sounds without hearing them at that time. Just what the relationship is between the neurophysiology of imagined or recalled sound and the neuorphysiology of hearing is an interesting question for the scientists to answer.

There are several different questions you may be raising here. One is how we can imagine something that doesn't exist, like a unicorn: aren't we then saying the same thing is both real and unreal? The short answer is that they are not the same thing: unicorns are not real, but imaginative experiences of unicorns are. Another question is how a physical brain can contain or even generate experiences. This is the general mind-body problem: a tough nut to crack. And there is also a third question. Even if we don't worry about how the mind can have experiences when there is an external cause -- say the experience of a horse when we look at one -- we might worry about how it has the power to generate an experience of something is has never seen, like a unicorn: where does this content come from? This too is an interesting question. Some philosophers answer that we make things up in our imagination by putting together bits and pieces of what we have seen -- say combining the general shape of a horse with the...

We all believe that mental processes influence physical outcomes. For example, right now my mental processes are (I hope) influencing the movement of my fingers on the keyboard. How is that possible? Well, if mental processes just are physical brain processes, then there is no special mystery. If on the other hand they are non-physical, then there may be a special problem in seeing how such different sorts of things -- the mental and the physical -- could be causally related. But I'm not even sure there is a special problem in this case. It's difficult to understand causation even in the simplest physical-physical case, but it is not clear that a big difference between cause and effect raises a special additional problem. After all, those moving fingers of mine are causes all sorts of changes to the world wide web, a digital realm very different from my digits. It's true that the expressions 'the power of positive thinking' and 'self-fulfilling prophecies' are cliches, but it seems...

For a machine to have knowledge, it looks like it has to be able to have beliefs, since when you know something you also believe it (though not conversely). And for a machine to have beliefs, it has to be able to form representations of how the world might be. So the answer to your question will depend in part on whether machines can form representations. This is a hotly debated question in the philosophy of mind, for the case where the machines in question are computers. The two most famous arguments are due to Alan Turing and to John Searle. Turing argued that there could be a computer that is able to engage in an extended intelligent conversation (by email perhaps) so good that it fools people into thinking it is a person, and that such a computer ought to be taken to have representational states. Searle argued that since we know how computers (traditional ones, anyway) actually create their end of the conversation, and that this involves only registering the electronic equivalent of the...

You are right. If you and I each assumes that our own thoughts are all the thoughts there are, then we are both wrong. Of course if I am really the only thinker, then my assumption would be correct, but it does not look like I could justify that assumption, since if you were out there too, you would be in just the same situation as I am. The way out of this, I think, is to see that the point is not that I have reason to believe that there are no thoughts other than my own, but that my own thoughts are the only ones I can be completely sure about. However much experience and however many thoughts I have, it is still possible that there is nobody else out there. Of course we all believe (rightly, in my view!) that there are other people and their thoughts out there, but the puzzle you have asked about encourages philosophers to try to work out how we can know this.

You are right that it is no argument for dualism that brains can do things that single neurons can't. The most powerful case for dualism is probably the enormous difficulty in seeing how facts about conscious experience could be purely physical facts, however complex. There is no fundamental conceptual difficulty in seeing how bits of silicon and wire could be put together in a way that yields a computer that can beat humans at chess; but it's much harder to see how those bits could be put together to generate conscious experience. Even that, however, is not the big problem, because many dualists would hold that mental states are caused by physical states. The big problem is seeing how the conscious state could itself be (not just be caused by) something entirely physical. This is the basis for the controversial and stimulating Knowledge Argument for dualism, due to Frank Jackson. No matter how much a deaf person knows about the physical properties of the sensation of sound, indeed even if he...

You're right: the fact that you seem to remember something doesn't mean it really happened, even if what you seem to remember is your own past thought. And it is not as if you can go back to check. But you can still at least sometimes evaluate the reliability of a particular memory. How plausible is it that the sort of thing I seem to be remembering would happen? How does it fit with other things I believe (even if almost all of those other beliefs are also based on memory)? These are the sorts of checks we run regularly, to decide when to trust our memory and when not to trust it. But it is difficult to see how to block the extraordinary doubt that pretty much all our memories might be wrong. As Betrand Russell pointed out, there seems to be nothing impossible about the idea that we and the rest of the world only came into existence five minutes ago, with our minds pre-stocked with a full set of false memories. That's skepticism for you.

According to property dualism, there are no non-physical substances, but there are non-physical properties, such as the property of having a certain kind of experience. So just as a physical property of a brain, say its mass, is located in the same place the brain is, so the property dualist may be able to say that a non-physical property of a brain is located there too.

This reminds me of that non-philosophers model of a philosophical question: If a tree falls in the forest and there is nobody there to hear it, does it make a noise? I've never been entirely sure just what question this was asking, but maybe it is about the nature of sound. According to some philosophers, sound is just an experience. In that case the tree would have made no noise, since there was no experience. Others would identify the sound with the vibrations in the air caused by the tree hitting the ground. In that case, the tree makes a noise whether or not there is someone to hear it (though not if there is no air around it). Anyway, back to your dream. A dream is a course of experience, and its existence doesn't depend on whether you remember it later. So you really can have a dream you don't remember, though if you don't remember it then you presumably won't know that you had it. It is the same with an experience you have while you are awake. If you heard a car backfire yesterday...

Much of our mental life is involuntary. For just one example, we can't straightforwardly decide what to believe. Thus if you don't believe p and I offer you a big reward if you start believing p, you can't just do it for the money. So it's not suprising that we can't stop thinking about X just because we want to. But the desire not to think about X may be worse than ineffective: it may actually be counterproductive. When we think about not wanting to think about X, that brings X to mind, so it sometimes has the opposite of its intended effect.