https://philosophy.as.uky.edu/sites/default/files/Brains%20in%20a%20Vat%20-%20Hilary%20Putnam.pdf ? ? ? ? ?(Chapter 1 of?Reason, Truth and History by Hilary Putnam)

https://plato.stanford.edu/entries/skepticism-content-externalism/#PutnArguAgaiBIVSkep

1. Skeptical Hypotheses and the Skeptical Argument

The Cartesian Skeptic describes an alleged logically possible scenario in which our mental lives and their histories are precisely the same as what they actually are, but where the causes of the facts about our mental lives are not the kinds of events in the external world that we commonly think they are. On Descartes’ Evil Genius hypothesis, there is no physical world. Rather you are a disembodied mind, and your entire mental life, with all of its experiences, has been caused by an all-powerful, purely spiritual Evil Genius. As a result, your beliefs about the external world, such as that you have a body, or that there are planets in the solar system, are all mistaken. On the brain-in-a-vat hypothesis, a given person is a disembodied brain living in a vat of nutrients. The nerve endings of the brain are connected to a supercomputer, whose program sends electrical impulses that stimulate the brain in the same way that actual brains are stimulated when perceiving external objects. If you are a brain in a vat, suppose, your conscious experiences are and have been qualitatively indistinguishable from the experiences you have had over the entire course of your mental life. But if as the result of your computer-caused experiences you believe, say, that you have a body, then you are mistaken.

Given such an hypothesis as that of the brain-in-a-vat, the skeptic can go on to argue that there are many commonsense beliefs that we think we know, but that we do not in fact know. One common such argument is based on the widely endorsed closure principle that?knowledge is closed under known entailments:

• (CL)For all persons?S?and propositions?p?and?q, if?S?knows that?p, and?S?knows that?p?entails?q, then?S?knows that?q.

Now pick any proposition about the external world that you think you know to be true but that is inconsistent with your being a brain in a vat, say, the proposition that you have a body. Then the skeptic can argue as follows:

• (SA)(1)You know that the proposition that you have a body entails that you are not a brain in a vat. [Premise]

• ∴ (2)If you know that you have a body, then you know that you are not a brain in a vat. [By (1) and (CL)]

• (3)But you don’t know that you are not a brain in a vat. [Premise]?

• ∴ (4)You don’t know that you have a body. [By (2) and (3)]?

Premise (3) seems justified by the fact that you have the same conscious experiences whether you are a normal human in a normal physical world or a brain in a vat. For again, the evidence you have for each alternative is exactly the same. So you don’t know that you’re not a brain in a vat.

2.?Putnam’s Argument Against BIV-Skepticism

On Putnam’s version of the brain-in-a-vat hypothesis, the contents of the universe now and always have been relatively few. There have existed and now exist only brains in vats of nutrients and the supercomputers that send and receive messages to and from each brain. Putnam’s hypothesis assumes that each human (and each sentient creature) that exists or has ever existed is one of the brains in the vats. The supercomputers are so clever that their electronic interactions with the brains result in exact duplications of the mental lives and histories of each person whose brain is in a vat. Thus I, you, indeed?we?(all human beings) are brains in a vat on this hypothesis. Let us call a brain in a vat of this sort a ‘BIV’.

Putnam’s goal is to refute the skeptical argument that is based on the BIV hypothesis, by providing a counterargument for the conclusion that we are not BIVs. Thus, each of us is provided with a way of?knowing?that she is not a BIV, contrary to premise (3) of the skeptical argument (SA) above. The crucial move in Putnam’s argument is a sub-argument to the effect that that if one were a BIV, one would not be able to grasp the meanings of various general terms, such as ‘tree’, ‘brain’, and ‘vat’. For in order for an utterance of ‘tree’ for example to represent or refer to trees, Putnam maintains, there must be some?causal connection?between uses of ‘tree’ and real trees (Putnam 1981 [1999: 37]). Following Wright (1992: 71), I will call this ‘the causal constraint’. A similar point, Putnam assumes, holds for the words ‘brain’ and ‘vat’, as well as for mental utterances of, and the concepts expressed by, such words. It follows that if we are BIVs, then since our uses of ‘brain’ and ‘vat’ do not bear the right causal relations to actual brains and vats, we cannot so much as?think?or?say?that we are brains in a vat (Putnam 1981 [1999: 31]).

Putnam goes on to suggest that while a BIV cannot think or say that she is a brain in a vat, her utterances of ‘BIV’ could refer to something?else. For instance, the utterances could refer to (i) BIVs-in-the-image. That is, the utterances could refer to the succession of experiences as of being a BIV. In this case, the BIV’s utterance would just be?false, since the computer program would never allow her to have perceptual experiences as of being an envatted brain as opposed to being an embodied person. (Of course, a BIV could only?seem?to be uttering words. So we should just identify her utterances with her mental ‘seemings’ to utter. See Putnam 1981 [1999: 31].)

In addition to ‘brain-in-a-vat-in-the-image’, Putnam suggests (1981 [1999: 36]) two other possible sorts of things that ‘brain in a vat’ might refer to for a BIV. It might also refer to (ii) the electronic impulses that cause experiences as of being a BIV or perhaps to (iii) features of the computer program that cause those electronic impulses. I will use the expressions ‘brain* in a vat*’ and ‘BIV*’ to provide a schematic representation of whichever of these three possible meanings of ‘BIV’ in vat-English is correct, or if none is correct, ‘brain* in a vat*’ and ‘BIV*’ represent the meaning of ‘BIV’ in vat-English, whatever it is.

The important fact for Putnam’s argument is that whichever of these possible meanings is had in vat-English by ‘I am a brain in a vat’, that sentence, as used by a BIV, will be?false. We saw that in sense (i), ‘I am a brain in a vat’ would be false in vat-English, since a BIV would never have (perceptual) experiences as of being a BIV. Putnam does not explain why ‘I am a brain in a vat’ would be false in both senses (ii) and (iii). Brueckner (2016: 4) suggests that since in sense (iii) both ‘brain’ and ‘vat’ would refer to certain computer program features, a BIV would not be a BIV in sense (iii): no BIV would be a certain computer program feature that is “l(fā)ocated in” a certain other computer program feature. A similar point would hold for sense (ii). From these considerations, Putnam then concludes: “In short, if we are brains in a vat, then ‘we are brains in a vat’ is false. So it is (necessarily) false” (Putnam 1981 [1999: 37]). Here, Putnam seems to think that he has shown the conclusion that he was aiming to show, namely, that?we are not brains in a vat. But in fact, the most he has shown so far is that a BIV’s utterance of ‘We are not brains in a vat’ would be true. But this conclusion is simply irrelevant to Putnam’s goal, since as Putnam himself points out, the BIV’s utterance?does not mean?that we are not BIVs (see Brueckner 1986: 152 and Forbes 1995 [1999: 63]).