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).


Zera Yacob (1599-1692), an Ethiopian philosopher – 21/03/2016

How many sub-Sahara African philosophers from before the 20th century do you know? I know two: Zera Yacob and his student Walda Heywat. Each of them wrote one treatise of only a couple of dozen pages each. It is believed that the written history of sub-Saharan philosophy begins and ends there. Why is this the case? Why is there not more written philosophy around? There are several factors that might explain this, one of these factors being the dominance of oral history in Africa until colonial times, which means that wisdom was passed on in the form of stories from generation to generation, rather than in the form of academic treatises. Whatever the explanation, Zera Yacob’s treatise is one of the few we’ve got, so if you want to know what African philosophy was about back in the day, this is where you should start.

Zera Yacob lived in Ethiopia in the 17th century, and those of you who are familiar with European early modern philosophy (Descartes, for instance) will notice, when they read Zera Yacob’s treatise, that the philosophical questions he addresses are not particularly new. Like his European and Islamic colleagues, Zera Yacob wondered whether we can know God, and conclude God’s existence, by means of reason rather than faith or revelation. Like in Europe and the Islamic world, he chose the method of rational introspection over relying on authority or tradition.

Due to the very short length of the treatise and the familiarity with its philosophical message, Zera Yacob’s work may not be the first place to turn to for philosophers looking for depth or original philosophical questions. Yet, it is interesting to note how similar philosophical questions were important in Ethiopia as well as in Europe and the Islamic world, simultaneously. This gives offers the humbling realisation that modernity is not a distinct Western achievement.

A further interesting characteristic of Zera Yacob’s work is that it is not only a philosophical treatise, but also an autobiography. He reports how he fled from the king after refusing to take sides in a religious conflict between the local Copts and the Jesuit missionaries who had managed to convert King Susenyos. Zera Yacob consequently lived in exile in a cave for about two years, which he describes as a pretty nice time, away from violent and ignorant people, meditating on God and humanity in welcome solitude. Philosophy, for Zera Yacob, was not an academic interest but a matter of urgency and immediate relevance.

You can read the entire treatise, along with some comments, on this blog.

Will the sun rise tomorrow? The problem of induction – 01/02/2016

In his Treatise on Human Nature (1739), the philosopher David Hume argued that it is not reasonable to believe that the sun will rise tomorrow.

I wonder if Hume was always such a gloomy character. I for one wouldn’t be inclined to invite a friend to my party if he would preach to everyone that they should give up the belief that the sun will rise tomorrow. That’d kill the party mood.

But is Hume right? Here’s why he thinks it’s not reasonable to believe the sun will rise tomorrow.

Let’s first start asking the question: why do we believe the sun will rise tomorrow? Well, you might say, it has risen every day until now. Therefore, it is reasonable to believe that the sun will rise tomorrow as well.

But Hume doesn’t think so. He argues that induction cannot lead us to this conclusion.

What is induction?
Suppose you find a raven. It is black. You look for another raven. It is also black. You go on a trip through the country, looking out for as many ravens you can find. All ravens you see are black. At the end you’ve seen many ravens and all of them are black. Based on these observations, you draw the conclusion: the next raven I’ll see will be black. Is this a valid inference?

An inference is valid if (and only if) the truth of all premises guarantees the truth of the conclusion. In this case, there are many premises:

p1. Raven no. 1 is black
p2. Raven no. 2 is black
p3. Raven no. 3 is black

pn. Raven no. n is black
conclusion: Raven no. n+1 will be black.


But do these premises guarantee the conclusion? No. Even if you’ve seen 23786 black ravens, and none of another colour, it is still possible that the next raven is an albino raven, and therefore white. So the conclusion is not valid…

…unless you add another premise. The key premise you need to add to make the inference valid is: the future resembles the past. So you get:

p1. Raven no. 1 is black
p2. Raven no. 2 is black
p3. Raven no. 3 is black

pn. Raven no. n is black
pm. The future resembles the past.
conclusion: Raven no. n+1 will be black.


But remember that the conclusion only follows if the premises are true. Is the premise ‘the future resembles the past’ true? Well, you might say, the future has so far resembled the past, because during my entire trip, the next raven was black all the time. But then you get the same problem:

p1. At t1, the future turned out to resemble the past.
p1. At t2, the future turned out to resemble the past.

pn At tn, the future turned out to resemble the past.
conclusion: The next time, the future will also resemble the past.


This is, again, an induction, so you’ll need ‘the future resembles the past’ as a premise, again. But that leads to a fallacy of circularity: one of the premises is the same as the conclusion. Circularity doesn’t prove anything, so we can’t say with certainty that it’s true that the future will resemble the past. Therefore, we can’t use it in our induction about the ravens, and therefore we can’t conclude that the next raven will be black.

albino raven

But what have these ravens to do with the sunrise?

According to Hume, the same applies to the sun. Just because it has risen every morning, doesn’t mean we can conclude that it will rise again tomorrow.

Why should I care?

Science, and indeed our daily conduct, would be impossible if we didn’t rely on inductive reasoning. So what do we do now? We have a problem!



This video explains the above in a slightly different way.

More info on Hume’s philosophy can be found in this episode of BBC’s In Our Time.