We can’t have empirical evidence that rules out 4. — Michael

I disagree. When one demonstrates that BIV is physically impossible, scenarios 3 or 4 were never a logical possibility. What was conceptualize from actual functional brains was demonstrated to be false.

Just because one can say or imagine something does not make it possible.

But as a fictitious narrative, one does not need to worry about the support of empirical evidence.

The fantasy of humans as possible brains in a vat belongs to the period when AI was believed to be achievable by analysing how the brain (which was an analogy for a computer) expressed its intelligence.

It seems to me that period is gone. Now AI, for example, is focussed on 'What happens next'. And our model of the human is at present just such a creature: body and mind are integrated, and they behave in constant expectation of what happens next.

A brain in a vat seems to me like a ghastly horror movie opinion, thought up by people who think a lot, and live little.

Another interesting perspective regarding BIV is psychological.

There is a disorder that is called “Thought insertion.” Wikipedia defines it as such, “ Thought insertion is defined by the ICD-10 (International Statistical Classification of Diseases and Related Health Problems) as the delusion that one's thoughts are not one's own, but rather belong to someone else and have been inserted into one's mind. The person experiencing the thought insertion delusion will not necessarily know where the thought is coming from, but makes a distinction between their own thoughts and those inserted into their minds.”

Even more fascinating is the therapy. Again from Wikipedia, “In other words, the patient would speak his thoughts out loud in order to re-give themself the feeling of agency as he could hear himself speaking and then contributing the thought to himself.”

I wonder if such metaphysical reasoning around BIVs could bring on such a disorder as Thought Insertion.

How does it show that you’re in 2 and not 4?

I disagree. When one demonstrates that BIV is physically impossible, scenarios 2 or 4 were never a logical possibility. What was conceptualize from actual functional brains was demonstrated to be false.

Just because one can say or imagine something does not make it possible.

But as a fictitious narrative, one does not need to worry about the support of empirical evidence. — Richard B

The thing is, if the fact is that you are a brain in a vat, who has been fed all of your perceptions by a mad scientist, then your belief that a brain in a vat is physically impossible is a result of the way your beliefs developed in response to what the mad scientist has been feeding you. So under this scenario you believe what the mad scientist caused you to believe, and so your belief that a BIV is physically impossible doesn't have an informed basis, and would in fact be false.

This is not to say that it is metaphysically possible that you are a BIV , but how could you justify the proposition that it is epistemically impossible?

3. and 4. are incoherent; if we were brains in vats then there could be no evidence since everything would then be an artificial construct. What is the brain in vat experience based upon? Must the vat and brain themselves be real in a real physical world? The brain in vat experienced modeled upon some real world experience?

since everything would then be an artificial construct — Janus

Yeah, we would experience fabricated evidence that either brains in a vat are possible or aren’t.

The point is that we can’t know whether or not what we see is real or the product of a scientist’s (or evil demon’s) manipulation. So to use what we see as evidence that brains in a vat are impossible is to beg the question and assume that we are not brains in a vat and so can trust what we see.

If the answer is impossible to determine, even in principle, in the brain in a vat scenario, or indeed even in our own presumed scenario of a real physical world, then the whole thought experiment would seem to be a pointless exercise. The "evil demon" can mean nothing to us unless he shows himself; otherwise he must remain an impossible figment.

Sure, but according to the metaphysical realist the truth is independent of any kind of falsification or verification, and so even if something like brains in a vat are impossible in principle to prove or disprove, it is nonetheless either true or false.

And that’s precisely why Putnam’s argument is an attempt at refuting metaphysical realism. If metaphysical realism is true then we might be brains in a vat, but we can’t be brains in a vat, and so therefore metaphysical realism is false.

What if it is not merely not the case, but is also physically impossible that we could be brains in a vat? That would not be incompatible with metaphysical realism. Logical possibilities are not informative and they undetermine both belief and knowledge in that they don't tell us anything interesting about the world; since they merely consist in anything we can imagine which is not self-contradictory.

Why did you say we can't be brains in a vat rather than saying that if we were, we could not, even in principle, know that we were?

What if it is not merely not the case, but is also physically impossible that we could be brains in a vat? — Janus

They might be physically impossible, but if we are brains in a vat then we cannot trust our experience to show us what is physically possible, and so to use what we experience as evidence that brains in a vat are impossible is to beg the question and assume that we are not brains in a vat and so can trust what we experience.

I'm not suggesting that we can know whether we are brains in vats, but merely that it might be physically impossible, even though we of course cannot know that. We can think that there might be physical impossibilities, and that would be entirely compatible with metaphysical realism, but we cannot know for sure what they are in any case.

So, we cannot know we are or are not brains in vats, but we can say that it seems way less plausible, and is a conjecture motivated only by it's being a logical possibility, which doesn't mean much. And in any case whether we are real physical beings or brains in vats, since the truth is unknowable, makes no difference to our lives as lived.

his is not to say that it is metaphysically possible that you are a BIV , but how could you justify the proposition that it is epistemically impossible? — wonderer1

How does it show that you’re in 2 and not 4? — Michael

1. Simply, if the scientist showed that it is physically impossible to have a functional BIV, BIV is not possible. This demonstration might be carried out by use of experiments in which actual brains are studied. These would be the same kind of brains we have studied empirically to understand the role they have biologically in the interaction with the world. This knowledge of the brain is then used by philosophers to construct their thought experiments based on the idea of how brain's function. But if philosophers want to make a claim that the brain placed in an artificial enviorment could function in such a way that would produce simulations, they are obligated to submit themselves to the scientific criticism that comes with the territory.

2. The problem I am seeing here is the philosopher wants to talk about ideas but they also want it to be about the world. So they pull a fast one, they talk about real world objects like brains and vats, accept our empirical understanding of the function of the brain, then use this language to come up with imaginary experiments and suggest all sorts of fanciful outcomes that they claim could really be true.

3. Consider this scenario: An individual claims they have designed a machine that can create energy. When he plugs the machine in he claims that when 100 Joules is inputed to the machine, he will get 200 Joules as output. He claims this is possible based on his design. He plugs it in and the output was 50 Joules. He says to himself, "I guess it was not possible for this machine to create energy." Upon hearing this, a scientist chimes in and says, "this was never possible because it would have violated the law of conservation of energy." Just because you could imagine a design of machine with potential outputs of energy, does not mean it is possible to be realize in the actual world. But some want to insist in saying, "but it is logically possible!" I want to say it is not possible at all but feel free to imagine what you like.

4. I like to say that the idea of BIV is vague at best or nonsense at worst. Basically, what is the scientist suppose to figure out if the idea is too vague and/or nonsense? If you say the scientist is suppose to create a lifetime of a human simulated in the brain, what does that mean? Who are you suppose to talk with to understand this request? If you say the scientist should create an artificial world exactly like our real world, are you comparing an inner world that was pick out by inner ostensive definitions to some external world? But how does this occur? Are we not moving in the territory of what Wittgenstein calls grammatical fictions?

5. Our understanding of the world comes from our use of language in a human community. We have common expressions/reactions to the world. We make similar judgments about objects and events in the world. And language is our vehicle to understand this world. All of this is accomplished through our interactions with other human beings. It is not by observing neuron activity in brains of our fellow human beings. The BIV treats brains like tape recorders, grey matter is the material we record bits of information and play back when we feel like. We don't believe tape recorders, film projectors, computers have rich inner lives, nor should we believe the grey matter in the BIV would also.

The thing is, if the fact is that you are a brain in a vat, who has been fed all of your perceptions by a mad scientist, then your belief that a brain in a vat is physically impossible is a result of the way your beliefs developed in response to what the mad scientist has been feeding you. So under this scenario you believe what the mad scientist caused you to believe, and so your belief that a BIV is physically impossible doesn't have an informed basis, and would in fact be false. — wonderer1

You are saying that a BiV brain is different than a real brain, I think. But then tell me this, is BiV perception the same as real perception?

You are saying that a BiV brain is different than a real brain, I think. But then tell me this, is BiV perception the same as real perception? — NotAristotle

Yeah might as well be flexible and say we can't know what a BIV actually is, but under the scenario what you think of as real perception would necessarily be the same as BIV perception.

Simply, if the scientist showed that it is physically impossible to have a functional BIV, BIV is not possible. — Richard B

What scientist are you referring to? Under this scenario your belief in scientists would be a function of what the mad scientist (god to you) is feeding you in the way of perceptions, so any beliefs about brains that you have would be a function of the virtual reality presented by the mad scientist tending your vat.

The mad scientist might have fed you sensations that resulted in you having a notion of a brain that is utterly unlike what is in the vat. You don't have knowledge of what is in the vat or even the physics of vat world, so you can't have a scientific proof of the impossibility of the thing in the vat in vat world.

Yeah might as well be flexible and say we can't know what a BIV actually is, but under the scenario what you think of as real perception would necessarily be the same as BIV perception. — wonderer1

That sounds right to my ear wonderer1; BIV perception would have to be the same as real perception, given that BIV perception is the same neuronal activity as real perception.

I will summarize our discussion as it appears to me so far. We both agree, I think, that if it is physically impossible in the real world to put a brain in a vat and simulate reality, then BiV is impossible. But your point of contention is to say that it may be possible to put a brain in a vat and simulate reality in the real world, although a brain in a vat may falsely think that BiV is impossible. In such a case, a BiV brain would have to be constituted differently than a real brain, given that putting a BiV brain in a BiV vat is impossible, although putting a real brain in a real vat is possible.

Now, let us suppose that BiV world has some neuroscientists in it. That seems like a plausible assumption, unless your position is that BiV world cannot contain any neuroscientists. But if that were your position we would have proven that our world is not BiV world. So for now, we may suppose that BiV world contains neuroscientists, and these neuroscientists have a lot of knowledge about brains.

That brings me to the next question I would like to ask you: is BiV brain different than real brain by having more parts or by having fewer parts?

What scientist are you referring to? Under this scenario your belief in scientists would be a function of what the mad scientist (god to you) is feeding you in the way of perceptions, so any beliefs about brains that you have would be a function of the virtual reality presented by the mad scientist tending your vat.

The mad scientist might have fed you sensations that resulted in you having a notion of a brain that is utterly unlike what is in the vat. You don't have knowledge of what is in the vat or even the physics of vat world, so you can't have a scientific proof of the impossibility of the thing in the vat in vat world. — wonderer1

How about this scenario: I can imagine a witch that can cast a spell in which they make some poor soul believe that they are experiencing a world I which they are an autonomous agent thinking for themselves. Is this a logical possiblity? If you say no because it does not fit in with our ideas of causality and scientific world view, this can just be turned around by skeptic where they say "well that's because the witches's spell is making you think that." I would like to say that while this can be imagine it is not possible based on our understanding of causality and scientific world view. For the BIV example, the same goes, if we come to find that the BIV is physical impossible, meaning it can't function like we think, not only is it physical impossible, it would be impossible in general. Basically, experience and testing would show that the idea was ill formed. Whether BIVs, dreams, evil demons, witches, etc., any of these used to demonstrate some radical skeptical position is at its core faulty, useless thinking. This thinking resembles what authors do when they produce fictitious narratives, borrow from their real life experience and make up fantasy tales with no intention of claiming that they are talking about real life events. This type of philosophical thinking is doing the same, but with the delusional attempt in trying to potentially say something of the world we live in.

Wittgenstein's "On Certainty" said it best, "505. It is always by favor of Nature that one knows something."

This thinking resembles what authors do when they produce fictitious narratives, borrow from their real life experience and make up fantasy tales with no intention of claiming that they are talking about real life events. This type of philosophical thinking is doing the same, but with the delusional attempt in trying to potentially say something of the world we live in. — Richard B

Perhaps I see more value in considering thought experiments than you do? Einstein's thought experiments played an important role in human understanding of relativity theory. Suppose we consider the merits of thought experiments, as a technology for stimulating human minds to look at things from a different perspective?

Anyway, as far as my best guess regarding the world we live in. I don't see any reason to think that it is physically impossible for a BIV to exist. I don't have any reason for confidence that it will ever be technologically possible for humans to maintain a human BIV capable of philosophical conversations 'in a vat'. Furthermore, if in the future the scientific knowledge and technology necessary to maintain a human brain in a vat is available, I'd expect those with the scientific know how and access to the technology would consider creating a BIV to be a silly (and possibly immoral) thing to do.

If we want to talk in more realistic terms, about what sort of technological minds humans might create, we might look at using a technology like spintronic memristors used to instantiate artificial neural networks.

Wittgenstein's "On Certainty" said it best, "505. It is always by favor of Nature that one knows something." — Richard B

Well, I'd quibble over whether using the word "favor" is saying it best. I'm inclined to say something more like, "It is always as a consequence of interactions occurring in Nature that one knows something."

Perhaps I see more value in considering thought experiments than you do? Einstein's thought experiments played an important role in human understanding of relativity theory. Suppose we consider the merits of thought experiments, as a technology for stimulating human minds to look at things from a different perspective? — wonderer1

The role of imagination in scientific theorizing is not in question. Also, I certainty would not say that philosophy cannot offer insights to a scientist. In nice article by John Norton, "How Hume and Mach Helped Einstein Find Special Relativity", provides a nice summary how these two philosophers, belonging to the empiricist/positivist traditions, influenced Einstein's abandonment of the idea of absolute time and simultaneity. However, even in this article, Einstein echoed what I have been saying. In section 3.1, titled "Concepts Must be Grounded in Experiences", he quotes Einstein, "‘Similarly,’ Einstein continued, ‘with the concept of simultaneity. The concept really exists for the physicist only when in a concrete case there is some possibility of deciding whether the concept is or is not applicable."

Well, what do you say wonderer1, is the BiV brain different than a real brain by having more parts or by having fewer parts?

wonderer1: I should rather think that a BiV brain cannot be compared to a real brain; they are completely different.

NotAristotle: As you say wonderer1, but then neither you nor I are a brain in a vat. We may be utterly deceived by our senses, but it is not by being a brain in a vat.

wonderer1: But I insist that we are brains in a vat; it must be so.

NotAristotle: Well then when we say brain we must mean something like a real brain.

wonderer1: Very well.

NotAristotle: Let us return then to our original inquiry: you seem to have said that a real brain is different than a BiV brain. But is it different by having more parts or fewer?

wonderer1: Surely it is by having fewer parts NotAristotle.

NotAristotle: Well put wonderer1, for if it had more parts, then it could be placed in a vat, just like a real brain. However, we are saying that it is impossible to place a BiV brain in a BiV vat. And this must be the case on account of a BiV brain having fewer parts than a real brain.

wonderer1: Indubitably.

NotAristotle: And what of our neuroscientists? Will they not be very much surprised to find that parts of our brains are missing?

wonderer1: Perhaps not.

NotAristotle: How do you mean wonderer1?

wonderer1: Perhaps the parts of the brain that are missing are not essential to perception, but are extra parts.

NotAristotle: Ah, I see, so these extra parts are excised, leaving behind the parts that are necessary for BiV perception.

wonderer1: Precisely.

NotAristotle: But if the parts that are necessary for perception are left intact, then once again, it should be possible to place a BiV brain in a BiV vat. But again, we have said this is impossible.

wonderer1: That is so.

NotAristotle: Then the problem arises once again, that our neuroscientists will likely find that parts of the brain are missing that are necessary for perception.

wonderer1: As you say NotAristotle.

NotAristotle: It stands to reason then, that if we are brains in a vat, the evidence will amount to saying that it should be impossible for us to perceive, given that we would be missing parts of the brain that are necessary for perception. In short, if we are brains in a vat, we will be missing parts of our brain, and neuroscientists will be quite perplexed by that.

wonderer1: Surely what you say is true NotAristotle.

NotAristotle: Well then wonderer1 that is good news. For, unless you and I are missing parts of our brains, we are most certainly not brains in a vat.

wonderer1: Good news indeed NotAristotle.