Hilary Putnam’s Semantic Externalism – 11/04/2016

Philosopher Hilary Putnam (1926-2016) passed away last month. Not only did he contribute to many debates in analytical philosophy, but he was also known to exhibit a great philosophical virtue: he was his own fiercest critic. He subjected his own positions to thorough scrutiny and was never too proud to reject earlier positions, when he had exposed their flaws, and change his mind in favour of another view.

In his honour, we will discuss one of Putnam’s arguments: his argument for semantic externalism. Crudely put, semantic externalism is the view that words and sentences only have meaning when there is an external world they can refer to. Putnam’s argument in favour of this view also serves to dismiss Cartesian scepticism. I will start from this vantage point to explain Putnam’s view.

Scepticism: Are we a brain in a vat?

Imagine you are, without knowing it, no more than a brain floating around in a vat of nutrient fluids. The brain is connected to a computer which stimulates the brain in such a way that it gives you the experience of an external world – you see trees, apples, other people, you smell flowers, you feel the fur of the cat and everything else you normally experience. These computer generated experiences are, to you, indistinguishable from experiences of real trees, apples, people, flowers and cats. You have been a brain in a vat from the beginning of your existence, and you have never known that you are a brain in a vat, because the computer has generated a whole life full of experiences for you. Therefore, you think that you see apples and feel the cat, but you are mistaken, because there are no apples and cats; there are only brains, vats, and computers (of which you have no experience). A bit like in the film The Matrix. If you’ve attended the class on Descartes, you’ll recognise his idea of a mind that is deceived by an evil demon. The brain-in-vat thought experiment is based on that idea.

This thought experiment aims to show that scepticism, the view that we can’t know anything about the world, is true. The argument goes as follows: if we know something about the world, then we can thereby also know that we are not brains in vats; but there is no way of knowing whether we are brains in vats, so we don’t know that we are not brains in vats; since we don’t know that we are not brains in vats, we must conclude that we don’t know anything about the world. (The logic applied here is a modus tollens: 1. if p then q, 2. not q, therefore not p.)

Putnam’s refutation

A friend of my housemate says that his worst fear is that we are indeed brains in vats, without ever knowing it. Luckily, Putnam has good news for him: we can know that we are not brains in vats. That’s a relief! But how does Putnam know? Putnam’s argument runs as follows:

Imagine a world in which the only existing things are brains, vats with nutrient liquids, and computers that stimulate the brains. Nothing else. Quite like the scenario outlined above. In this scenario, you are a brain in a vat (henceforth BIV) who has experiences as if you see trees, apples and cats, even though there aren’t any. You think you speak English, but this too is an illusion; you don’t speak at all (because you have no mouth or vocal cords, only a brain), it’s just the computer giving you the experience as if you speak English, you don’t actually speak. Hold that image for a minute. Let’s call this world the BIV-world.

Now imagine another world where you are not a brain in a vat. You are a human being in a natural environment, exactly as you experience it. You see trees, apples and cats and they are indeed really there. There are also other people who experience these same things, and such day-to-day experiences of the environment are not illusions. In this world, you also think you speak English, but this time, that is not a mistake: you do indeed speak English. In this world, when you use the word “tree”, the meaning of that word is, according to Putnam, caused by real trees in the world. The word “tree” refers to a tree, and that’s why it is a meaningful word in the English language. Let’s call this world the natural world.

Back to the BIV-world. In your mind, you’re speaking English. You use words like “tree”, “apple” and “cat”. But what do these words mean? There are no trees, apples or cats that can give these words meaning, because there are only brains, vats and computers. What the word “tree” actually refers to, in this case, is merely a set of electrical impulses generated by the computer. So the word “tree” in the BIV-world in BIV-English means something else than the word “tree” in English in the natural world.

So in the BIV-world, the words “brain” and “vat” refer, just like “tree” and “apple” to particular sets of electrical impulses, and not to the actual brain nor the vat it’s floating in in the BIV-world. So, paradoxically, even if you were a brain in a vat, your sentence “I am a brain in a vat” would be false, since “brain” and “vat” in that sentence don’t refer to your BIV-situation, but to illusory brains and illusory vats in your illusory environment.

I understand if your brain hurts a bit after reading this. Here’s the argument again, but this time in a schematic form. Concepts, references and words referring to objects in the BIV-world are indicated with an *:

  1. Either I am a BIV (speaking vat-English*) or I am a non-BIV (speaking English).
  2. If I am a BIV (speaking vat-English*), then my utterances of ‘I am a BIV’ are true if and only if (iff) I am a brain* in a vat*.
  3. If I am a BIV (speaking vat-English*), then I am not a brain* in a vat*.
  4. If I am a BIV (speaking vat-English*), then my utterances of ‘I am a BIV*’ are false.
  5. If I am a non-BIV (speaking English), then my utterances of ‘I am a BIV’ are true iff I am a BIV.
  6. If I am a non-BIV (speaking English), then my utterances of ‘I am a BIV’ are false.
  7. My utterances of ‘I am a BIV’ are false in either case.
  8. My utterances of ‘I am not a BIV’ are true.
  9. My utterances of ‘I am not a BIV’ are true iff I am not a BIV.
  10. Conclusion: I am not a BIV

If this hurts your brain even more, I understand. It’s all very technical. That’s the thing with analytical philosophers. They tend to do that. But the bottom line of Putnam’s argument is understandable enough: the brain-in-vat hypothesis mixes two worlds of reference, which messes with the meaning of what we talk about when we say “I am a brain in a vat”. If it were true that I am a brain in a vat, it would be meaningless because the concepts I use to talk about that situation don’t take their meaning from the BIV-world, but from the world I experience. Hence, the world I experience is the only world I can talk about, and in that world I am not a brain in a vat.

The Twin Earth thought experiment

Putnam makes the same point using a different thought experiment.

Imagine that somewhere far, far away in the universe, there is a planet that is an exact copy of Earth. Let’s call it Twin Earth. On Twin Earth, there is a twin of every person on Earth. So let’s say that Tony lives on Earth, and Twin Tony lives on Twin Earth. There is only one difference between Earth and Twin Earth: there is no water on Twin Earth. Instead, there is another substance with the exact same qualities as water, except that its chemical composition is not H2O, but XYZ (elements that don’t exist on Earth) Twin Tony, who speaks Twin English, calls this substance “water”. Now let’s go back a few hundred years in time, when nobody knew about the chemical elements and the composition of water. Would we say that Tony and Twin Tony mean the same thing when they say “water”? Putnam would say “no”, because Tony means H2O when he says “water” and Twin Tony means XYZ. The stuff they refer to is different stuff, even if it has the same function and Tony and Twin Tony have the same concept in mind. It is the actual things in the world that words refer to that give those words their meaning; language on its own, isolated from a world of reference, is meaningless, according to Putnam. In his own words: “‘Meanings’ just ain’t in the head”.

Of course, Putnam wouldn’t be Putnam if he hadn’t revised his position since. He later agreed with Tyler Burge that Tony and Twin Tony wouldn’t in fact have the same mental state when they talk about “water” (Tony has the concept of H2O and Twin Tony the concept of XYZ).


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