Jan Ardena: I think you're still missing the point of this philosophical discussion. Before I respond to your last post point-by-point, I'll try to set out the argument succinctly one last time for summary purposes. We all live our lives every day trusting that our sense impressions accurately reflect real things about the world. So, you believe you have a body, with a brain in your skull. You think that you see the world more or less as it is. You have human feelings such as love, and frustration and sadness and joy and all those other things. You consider yourself to be a more-or-less autonomous individual. The brain-in-a-vat picture exposes the fact that you can't actually know that any of these things that you take for granted is true in fact. Start with a simple example, more closely tied to what you call "the real world". When you look at something, you don't see the thing directly. Light comes into your eyes, and is turned into electrical signals, which are then processed by your brain. The last step in that process is you being consciously aware of a sense impression like "I can see an apple." So, the first thing to notice is that you have no direct perception of the apple. Your entire experience of the apple is filtered through your senses and through your brain. Now we ask the question: would your perception be any different if we were, for example, to remove the actual apple and instead stimulate the appropriate parts of your visual cortex to re-create that mental image you have of an apple in all its usual detail? It shouldn't be too much of a stretch to realise that your perception wouldn't be any different, especially seeing that something very similar happens when you watch a TV picture of an apple or a 3D projected image of an apple. Going a step further, what if we could replace all the sensory inputs to your brain using technology sophisticated enough to produce exactly the same sense perceptions as you have while you're walking around in the "real world". The "feed" to your brain could come from the real world, but it needn't. It could, for example, be simulated, like a virtual-reality projection. In this later case, would you know the difference between the "fake" sensory inputs and the "real" ones. Again, it shouldn't be a stretch to conclude that there's no way you could know the difference, given a good-enough simulation (or feed). Now realise that if we have managed to duplicate your sense impressions by wiring up your brain, then as far as you're concerned it doesn't matter where that brain is physically located any more. If we scooped your brain out of your skull and through a miracle of modern medicine managed to preserve it, wired-up, in a suitable vat of nutrients, you wouldn't notice any difference from having it your skull. If you want to go for one last step, why bother with the physical brain, or indeed the physical world, at all? A sophisticated-enough computer could, in principle, instead duplicate all of the functions carried out by your biological brain, and we could feed that computer the usual feed of sense impressions. But now, both you and the world as you know it exist entirely within a complicated and detailed simulation instead of in the "real world" that you perceive. You can call this a thought experiment or a game if you like. But here's the clincher: this scenario, or something like it, could be your reality, right here and now. In other words, there's no conceivable test you can carry out to distinguish "you" being a flesh-and-blood human being walking around in the world from "you" being a brain in a vat or a computer simulation of a person. You might object that a computer simulation of a human being is not a human being. That is an interesting discussion in itself, but somewhat beside the point here. The point is that, according to your lived experience, there would be no way to distinguish the two situations, even if there is a meaningful difference. Computer-simulated you would perceive all the same things as "real" you, think all the same thoughts, have the same loves and joys and sorrows that you have etc. It might even occasionally speculate about whether it could possibly be a brain in a vat. Just to emphasise: in the brain-in-a-vat scenario, it's very hard to argue that the brain in the vat wouldn't be "you". If your brain was in a vat, wired up for all the usual sense impressions that you normally have, that brain would be experiencing all the things you normally experience. It would think all the same thoughts you usually think. It would be you just as much as the flesh-and-blood brain you currently think you have is you. For religious people, of course, this whole idea that your brain is the essence of "you" seems unpalatable. There must be something more! So, as a religious person, you'd probably say that the "real" you is some kind of disembodied soul or spirit. Again, this is somewhat off-topic for the discussion, but we could ask what the nature of the connection of this soul or spirit to a physical body and brain is; this one is probably better left for a different thread. If you think the soul is important, then as far as the brain-in-a-vat scenario goes, the pertinent questions are: does the soul follow the brain, even if it is in a vat, or does it require an intact body to function correctly? Or would the soul depart if the brain was transferred to a vat? Can a soul only exist inside a flesh-and-blood human being? Could a computer-simulated person or a brain in a vat know that it didn't have a soul (if indeed it is the case that these things are "soulless")?