You could give your parents the argument that laziness is better than nothing, and nothing is better that than the most rewarding activity, so laziness is better than the most rewarding activity. But then again, that argument is fallacious. In fact, maybe your parents are right. But you might argue that staying with the cello only makes sense if you will really find it rewarding in the long term, and the fact that you do not find it rewarding after sixteen years of playing is powerful evidence that you will not find it rewarding in the long term. This argument hardly amounts to a proof, but at least it isn't fallacious.

It does seem possible that nobody else exists: I just have a very lively imagination. An interesting line of thought to the contrary is that my very ability to think contentful thoughts depends on the existence of other people. In that case, if I can have the contentful doubt whether other people exist, then they do. But can I prove that I have contentful thoughts?

David has given you two arguments for extra universes based on scientific considerations. These sorts of case are particularly neat because, as I understand it, we are physically cut off from other universes: there is no causation between worlds. But your ant analogy suggests that you also wonder whether there might not be more than we can sense or know in our own universe. There is indeed much more in our world than we can sense, and our scientific theories describe a lot of it. But is there much more in our world than we can know, once we take account of how far beyond what we can sense science takes us? To be unknowable, it seems that features of the world would have not just to be unobservable but causally isolated from what we can detect. (That is why David's examples were neat.) But there are many events in our world with which we cannot interact causally, given how limited is our own existence in spacetime. And even where there is causal interaction, I suppose that, like the ants, we...

In everyday life, we change our beliefs all the time. In philosophy, belief change is less common, because the beliefs in question are often very deep-seated and indeed in some cases -- for example the belief tables don't disappear when you blink -- effectively unrevisable. But if you want a personal example of philosophical belief-change, I used to think that it was fine to eat veal and ordinary (i.e. not free-range) chicken. Then I made the mistake of teaching a couse on applied ethics, and I changed my mind. Or if that seems too ordinary for you, I used to think that the content of my beliefs must be fixed by what is going on in my brain or mind. Then I read Putnam and Kripke...

This question is at the heart of David Hume's great sceptical problem of induction. He argues that there is no possible reason for saying that the laws of physics won't change overnight, since to say this would be to make a prediction, and our method of prediction just presupposes that this won't happen. Put another way, it looks like you can't have evidence that the future will be like the past, because all your evidence is in the past, so to use that to show something about the future would require that you already know whether the future will be like the past. There has been a great deal of productive work on the problem of induction, but nobody has come close to a full solution. If you want to get into this great issue, the best place to start is Hume's classic discussion in his Enquiry Concerning Human Understanding , especially sections IV and V.

You can't think without thinking, but fortunately it doesn't follow that you can only think about thinking. There are wonderful philosophical questions about how we perform the feats of representing things in the wolrd and of distinguishing correct from incorrect representations; but the the undeniable fact that all our mental representations are indeed mental does not show these feats to be impossible.

Also because you want to know what the world is like (even when your life doesn't depend on it) and only true propositions tell you that. To have any beliefs at all is to manifest an interest in the truth and falsity of propositions.

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.

What are the differences between wrong and ignorant? I guess that you can be ignorant without being wrong, since ignorance may involve having no idea what the answer is. And perhaps you can be wrong without being ignorant, if nobody is in a position to know the right answer, or if say your error is the result of a mistake in a difficult calculation. But maybe the contrast you have in mind is brought out by someone who is ignorant by being wrong in a case where he or she should have known better. Let's go with that. The next challenge is what 'world view' means. Let's suppose that it means some broad philosophical position, like materialism/dualism, realism/anti-realism, realism/nominalism, consequentialism/deontology, etc. In that case, I would say that even if I am convinced that my philosophical position is correct and that yours is therefore wrong, I should not normally say you are ignorant. Big philosophical positions are controversial, they are things over which highly intelligent and...

I agree with Alex that animals are not morally accountable: they cannot be morally blameworthy (or praiseworthy). What is harder to explain is why we don't have a responsibility to prevent animals from hurting each other. This is an unnatural thought, yet perhaps technically we would have this responsibility, if we could also find an alternative diet for the natural carnivores. But given how awful is our own treatment of animals, we should perhaps concentrate on getting our own act together first.