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In class we discussed the famous "brain in a vat" case, where since you can't differentiate between that case and "reality", you have no way of knowing that you are, in fact, in reality. But what I don't understand is, why is it a meaningful scenario? Isn't reality just what we perceive anyway? If I have no way of knowing that I'm just a brain in a vat, and I never will, then how does this make what I see as "reality" any less real?

  • The 'brain in a vat' is a skeptical hypothesis. Philosophical skepticism uses skeptical hypotheses to question the possibility of certainty in knowledge. The usefulness of BIV (brain in a vat) lies more in what it shows (no certainty in knowledge) rather than what it implies (you being a brain in a vat). – WaWaWa Nov 5 '14 at 8:05
  • Right. But my point is that it doesn't actually imply any lack of certainty in knowledge, since it simply means adding the caveat "in reality as I perceive it" to anything I think I know, thus - "in reality as I perceive it, it's not raining". Now, since reality as I perceive it is all that exists, what would it matter if I was just a brain in a vat? – ewkochin Nov 5 '14 at 8:12
  • The point of the argument is not whether you are a brain in a vat (and whether it matters) but whether you can know if you are a brain in a vat or not. This is the skeptical argument: I do not know that not-P.... If I do not know that not-P, then I do not know S.... I do not know that S. BIV serves as the skeptical hypothesis, a starting point (a thought experiment) where you cannot (and are not allowed to) know if not-P or P is the case. – WaWaWa Nov 5 '14 at 8:39
  • @ewkochin It is a very modest account of existence that reality as you perceive it is all that exists. Some might challenge that. Your perception might be fooled by something that you are unaware of. With you account, colors would seize to exists when everyone turn colorblind. If someone proposes: "What if there were colors, and we are all colorblind?" you would ask here "How is that scenario meaningful if perception and reality are congruent? We will never know if there are colors anyway!" – Einer Nov 5 '14 at 11:11
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    Ewkochin, what @Einer shows is that there are philosophers who would disagree with your idea that everything beyond perception is uninteresting. The difference between the two is universality, which is essential in human interaction. – Keelan Nov 15 '14 at 7:46
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In class we discussed the famous "brain in a vat" case, where since you can't differentiate between that case and "reality", you have no way of knowing that you are, in fact, in reality. But what I don't understand is, why is it a meaningful scenario? Isn't reality just what we perceive anyway?

I'm not a fan of the brain in the vat thought experiment because it is rather silly and understates the significance of many important problems. However, your argument against it is not much good.

There are two problems with the claim that reality is what we perceive. First, many scientific explanations invoke things that nobody has ever perceived and that perhaps nobody ever will perceive. For example, nobody has seen the core of the sun. Maybe nobody ever will. Nobody has ever seen a dinosaur, only dinosaur skeletons. But in both instances there are many explanations about those unperceived entities. There is no way of explaining how the world works without them, so as far as we know they are real.

Furthermore, your perceptions are not just showing the world the way it is. Rather, they introduce many layers of interpretation. This is what gives rise to optical illusions. Your visual system makes assumptions about the way the world works, so if you know those assumptions you can produce misleading effects by making suitable images or models.

Reality as we know it is not what we perceive, nor is it derived from sense perception. Under some circumstances we can interpret what we see as true and other circumstances we know better. So under some circumstances we disregard what we see in light of an explanation. Rather our knowledge about how the world works comes from making guesses and then criticising those guesses. Sense perception can help us criticise guesses, but we can't know what the world is like just by looking at it. For more discussion of this issue see "The Beginning of Infinity" by David Deutsch, chapter 2. See also Chapter I of "Realism and the Aim of Science" by Karl Popper.

If I have no way of knowing that I'm just a brain in a vat, and I never will, then how does this make what I see as "reality" any less real?

We do know that we are not brains in vats. The world is extremely complex. If we were just brains in vats there would have to be some hardware running some very complex simulation. The only difference between the brain in vat explanation and the true explanation is the the brain in vat explanation slaps a label on everything saying it is a simulation. This label explains nothing about how the world works and so it is an unexplained complication of our existing explanations and should be rejected. See "The Fabric of Reality" by David Deutsch, Chapter 4.

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I feel that whether or not we know the truth, the truth still exists is one of the essential points of BIV. It is a starting point. You suggested that we all need to prepend "in reality as I perceive it" to the beginning of any statement as an nonessential caveat. BIV is suggesting that it is an essential caveat and that we need to continually maintain that perspective as annoyingly uncertain as it is, lest our ego's trick us into maintaining or arguing for less encompassing paradigms like Newtonian physics vs. Einsteinium physics or shadows on the wall as opposed to reality.

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