The “Brain in a Vat” thought experiment is an update to René Descartes’ evil demon problem. Hilary Putnam is credited with this update. The example supposes. Brains in a vat. An ant is crawling on a patch of sand. As it crawls, it traces a line in the sand. By pure chance the line that it traces curves and recrosses itself in. In a famous discussion, Hilary Putnam has us consider a special version of the brain-in-a-vat.

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The skeptical hypothesis that one is a brain in a vat with systematically delusory experience is modelled on the Cartesian Evil Genius hypothesis, according to which one is a victim of thoroughgoing error induced by a God-like deceiver. The skeptic argues that one does not know that the brain-in-a-vat hypothesis is false, since if the hypothesis were true, one’s experience would be just as it actually is. Therefore, according to the skeptic, one does not know any propositions about the external world propositions which would be false if the vat hypothesis were true.

Hilary Putnam provided an apparent refutation of a version of the brain-in-a-vat q, based upon semantic externalism. This is the view that the meanings and truth conditions brwins one’s sentences, un the contents of one’s intentional mental states, depend upon the character of one’s external, causal environment. This entry is primarily focussed upon evaluating the Putnamian considerations that seem to show that one can know that one is not a brain in a vat.

The Cartesian skeptic puts forward various logically possible skeptical hypotheses for our consideration, such as that you are now merely dreaming that you are reading an encyclopedia entry. The more radical Evil Genius hypothesis pufnam this: In the Evil Genius world, nothing physical exists, and all of your experiences are directly caused by the Evil Genius.

Brain in a vat

So your experiences, which represent there to be an external world of physical objects including your bodygive rise to systematically mistaken beliefs about your world such as that you are now sitting at a computer. Some philosophers would deny that the Evil Genius hypothesis is genuinely logically possible. Materialists who hold that the mind is a complex physical system deny that it is possible for there to be an Evil Genius world, since, on their view, your mind could not possibly exist in a matterless world.

Accordingly, a modern skeptic will have us consider an updated skeptical hypothesis that is consistent with materialism. Consider the hypothesis that you are a disembodied brain floating in a vat of nutrient fluids. This brain is connected to a supercomputer whose program produces electrical impulses that stimulate the brain in just the way that normal brains are stimulated as a result of perceiving external objects in the normal way. If you are a brain in a vat, then you have experiences that are qualitatively indistinguishable from those of a normal perceiver.

If you come to believe, on the basis of your computer-induced experiences, that you are looking at at tree, then you are sadly mistaken.

After having sketched this brain-in-a-vat hypothesis, the skeptic issues a challenge: Do you know that the hypothesis is false? The skeptic now argues as follows. Choose any target proposition P concerning the external world, which you think you know to be true:. Premise 1 is backed by the principle that knowledge is closed under known entailment:. Premise 2 is backed by the consideration that your experiences do not allow you to discriminate between the hypothesis that you are not a brain in a vat but rather a normal human from the hypothesis that you are a brain in a vat.

Your experience would be the same putnzm of which hypothesis were true. So you do not know that you are not a brain in a vat. In a famous discussion, Hilary Putnam has us consider a special version of the brain-in-a-vat hypothesis.

Imagine that you are a brain in a vat in a world in which the only objects are brains, a vat, and a laboratory containing supercomputers that stimulate the envatted brains. Imagine further that this situation has arisen completely randomly, and that the brains have always been envatted. No evil neuroscientists or renegade machines have brought about the brains’ envatment. A skeptical argument just like that above can be formulated using the BIV hypothesis. Putting things now in the first person, Putnam argues that I can establish that I am not a BIV by appeal to semantic considerations alone — considerations concerning reference and truth.


This will block the BIV version of the skeptical argument. Here is how Putnam motivates his anti-skeptical semantic considerations. Suppose that there are no trees on Mars and that a Martian forms a mental image exactly resembling one of my tree-images as a result of perceiving a blob of paint that accidentally resembles a tree. Putnam’s intuition is that the Martian’s image is not a representation of a tree.

This is due to the lack of any causal connection between the image and trees even, we will suppose, any attenuated causal connection such as interaction with a visiting Earthling who has braibs trees. If I were a BIV, then my mental image resembling a tree would no more be a representation of a tree than would the Martian’s mental image.

Neither of us would have the sort of causal contact with trees which is required lutnam our images to refer to trees. Putnam offers three possibilities:.

On each of Putnam’s proposed reference assignments, though, the brain’s braisn token comes out true provided that the brain is indeed being stimulated so as to have experiences just like those a normal human has when seeing a tree and that the stimulation is caused by the appropriate electrical impulses generated by a computer’s program features.

Call these considerations about reference and truth semantic externalism. This view denies a crucial Cartesian assumption about mind and language, viz. His sentences express beliefs that are true of his strange vat ;utnam. The differences in the semantic features of the bgains used by the BIV and those used by his normal counterpart are induced by the differences in the beings’ external, causal environments. If DA succeeds, then we have a response to a skeptical argument involving the BIV hypothesis which shares the form of the Cartesian argument 1 – 3 above.

Thus we would have a response to the skeptic’s claim that since I do not know that I am not a BIV, then I do not know any target external-world proposition P. Step h itself follows from g on natural assumptions about negation, truth, and quotation, but T is problematic in the current anti-skeptical barins.

The assumption of T seems to beg the question against the skeptic. Putnam’s semantic externalist picture is this: So in putna, to know that T is the correct statement of my sentence’s truth conditions, I brainns to know that I am a non-BIV speaking English. But that is what the anti-skeptical argument was supposed brins prove. Let us consider two other reconstructions of Putnam’s thinking regarding BIVs.

We will discuss B below. Premise A comes from Putnam’s semantic externalism, as seen above. DA’s claims about the BIV’s sentences’ truth conditions are grounded in claims about reference such as A: We will discuss Vag below. Perhaps the vzt is something like this: SA2 highlights the connection between semantic externalism and the mind. Not only do meaning, reference, and truth depend upon one’s external environment in the ways we have discussed; further, the representational contents of one’s thoughts, beliefs, desires and other propositional attitudes also depend upon circumstances external to one’s mind.

The simple arguments are simpler than DA, and they also do not commit the anti-skeptic to a specification of the referents of the BIV’s words and the contents of its thoughts. The arguments rest only upon the claim that the referents and contents in question differ from my referents and contents.

Let us now turn to an objection to SA1.

Though the argument does not obviously require knowledge that I am a non-BIV speaking Englishas Supplemented DA seemed to, its premise B does seem upon reflection to be question-begging.

But a problem still remains. A similar worry can be laid at the door of SA2. In order to know its second premise, EI need to know what I am now thinking. So in order to know what I am now thinking in order to know that I am thinking that trees are greenit seems that I need to know that I am not a BIV thinking a thought with a strange content.


A reasonable response to the foregoing objection to Modified SA1 is as follows. But I do know certain things about my own language whatever it is and wherever I am speaking it.

This is a priori knowledge of semantic features of my own language whatever it is — English or vat-English. A similar response to the foregoing objection to SA2 is that I have knowledge of my own mind that is not experientially based. I can gain the knowledge that I am now thinking that trees are green via introspection.

Putting this self-knowledge together with my a prioriphilosophical knowledge of SA2’s first premise, Dknowledge based upon my understanding of semantic externalismI can then knowledgeably deduce that I am not a BIV. A problem for this response has been raised by various philosophers.

It has been suggested that semantic externalism engenders severe limits on self-knowledge: So the response we have considered may be in trouble if semantic externalism gives rise to such skepticism about knowledge of content.

“Brain in a Vat” Argument, The | Internet Encyclopedia of Philosophy

The foregoing defenses of the Simple Arguments emphasize a constraint on anti-skeptical arguments: The justification of their premises must not require any appeal to the deliverances of sense-experience. Now Vxt SA1 is driven by the following thought: This thought in turn rests upon the natural assumption that trees are not computer program features.

But is that assumption something that I know a priori? In work unrelated to bgains, Putnam has claimed that even though it is necessary that cats are animals just as it is necessary that water is H 2 Oit is not knowable a priori that cats are animals just as it is not knowable a priori that water is H 2 O. According to Putnam, the concept of cat allows that in vatt of gaining knowledge of their inner structure, cats could turn out to be robots.

The worry is that in a similar way, the concept of tree is such that in advance of gaining knowledge of the existence and vay of trees, trees could turn out to be computer program features. If I hold in abeyance my seeming a posteriori knowledge about trees, then, I cannot fairly say that in the vat world, there are no trees. This objection to Modified SA1 can be answered by focusing upon the dialectical situation between skeptic and anti-skeptic.

The skeptic wishes to impugn my seeming knowledge of the external world by putting forward a skeptical hypothesis that is incompatible with the external-world propositions I believe. On the current objection to our anti-skeptical argument, the skeptical critic undermines his own position by suggesting that SK is compatible with external-world propositions such as that I am in the presence of green trees. I can now argue as follows in response to the skeptic’s current objection.

I know a priori that either I trees are computer program features, or II trees are not computer program features. On the first alternative, the skeptic undermines his own overall position, and on the second alternative, the skeptic’s objection is withdrawn. So we could view Modified SA1 as being an argument by cases: Another objection to the semantic arguments we have considered springs to mind when we imagine a BIV working his way through, say, Modified SA1.

Understood in this way, his second premise is true. Thus, the BIV’s first premise is true in virtue of having a necessarily false antecedent since it is not logically possible for him to be a computer program feature. The following worry arises.

However, this worry is unfounded. Just read the argument carefully when you work through it! It bralns no difference to my argumentative situation if someone on Alpha Centauri uses those very sentences with different meanings from mine and proves that muons move rapidly.