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Chapter 4 Can We Trust Our Senses?

Part One: Descartes 1. Descartes’ Project {Side-heading: the historical context} Before the sixteenth century, few things could have seemed more certain than that the Earth is stationary. It seems so obvious. If you go to the top of hill and look out, you can see moving things (streams, clouds, animals etc.) but the Earth itself seems to stay still. This naïve argument was reinforced by the best science of the time: astronomers agreed that the Earth is stationary at the centre of the universe, while the stars, the planets, the Moon and the Sun move around the Earth, attached to transparent “celestial spheres”. Common sense and science were in agreement: the Earth does not move. This began to change in 1543 with the publication of Nicolaus Copernicus’s On the Revolutions of the Heavenly Spheres. Copernicus argued that the Sun is stationary at the centre of the universe, while the planets (including the Earth) move around it. Copernicus’s idea was slow to catch on, but by the early seventeenth century many scientists were convinced that Copernicus had been right. One of them was René Descartes (1596 – 1650), whose book Meditations on First Philosophy we’ll discuss in this chapter. {Side-heading: Descartes’ project} During Descartes’ lifetime, new scientific research undermined other medieval doctrines too. It had been generally believed that celestial bodies are perfectly spherical – a claim which Galileo Galilei (1564 – 1642) refuted by making careful telescopic observations of mountains on the moon. William Harvey (1578 – 1657) established that the heart pumps blood in a circuit around the body: out through the arteries, back through the veins. Most physiologists before Harvey thought that the liver creates blood; the heart then pumps it outwards to the different parts of the body; these various body parts are nourished by the blood, which is thereby used up. Descartes knew of all these developments in the sciences, and he had a hypothesis about why previous generations of scientists had got so many things wrong: they’d developed their theories with too much haste. Scientists in the past, Descartes thought, were guilty of jumping to conclusions. For example, their claim that the Earth is at the centre of the universe had not been demonstrated conclusively. Descartes came up with a stunningly ambitious plan. Thinking that there might be still more errors hidden in contemporary science, Descartes decided to start again – developing scientific theory entirely from scratch. He initiated this vast project in his book,

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Meditations on First Philosophy. In order to avoid mistakes like those of previous scientists, Descartes decided to put aside all pre-existing doctrine and to accept a claim only once he had shown that it was certainly true. He sometimes compared himself to a builder constructing a new building: he wanted the foundations of his building to be unshakeably solid. Descartes’ book has six chapters (or ‘meditations’). We’ll focus on the first three. We’ll finish with a brief discussion of Descartes’ influence. 2. The first meditation: searching for certainty {Side-heading: certainty is hard to find} Descartes wanted to find truths of which he could be certain – truths to form the foundation of his new science. But the more you think about it, the more you realize that certainty – complete certainty – is very hard to find. An example. You probably think that in New York City next year, July will be on average hotter than December. But can you be certain – completely certain, beyond all doubt – that some freak weather pattern won’t cause there to be snow in the city in the summer, when there would usually be flip flops and sunscreen? We think not. Weather prediction is probabilistic: near-certain or very confident is as good as it gets. Another example. You probably think that Notre Dame cathedral is in Paris. But are you completely certain? Here’s a story:

Last week The Walt Disney Company bought Notre Dame cathedral for an undisclosed though no doubt colossal sum. The iconic French building has been carefully disassembled and the pieces are now in transit to the Walt Disney World Resort. Once the pieces arrive in Florida, the cathedral will be rebuilt and used to house musical performances featuring famous Disney characters.

Can you be one-hundred-percent, not-a-shadow-of-a-doubt, bet-your-life-on-it certain that this story is mistaken? We think not. Strange things do occasionally happen, and even today we sometimes don’t hear about an important news story until a week or two after the event. Notice that we’re not claiming that this story is plausible or likely – it isn’t. It’s ridiculous. The point is that you can’t be completely certain that the story is false, and so you can’t be completely certain that Notre Dame Cathedral is in Paris.

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{Side-heading: the unreliability of sense-perception} Maybe it’s not surprising that we can’t be certain about next year’s weather, or about distant things like Notre Dame Cathedral. But it might seem reasonable to think you can be certain about what you can see right now. Descartes denied this. He pointed out that we’ve all been deceived by our senses in the past. For example, he noted that from a distance round towers sometimes look square. You may have noticed that in dim light coloured surfaces sometimes look grey. When capsaicin – an active ingredient in chilli peppers – is rubbed onto the skin, it can give the sensation of heat even when there is no change of temperature. Now Descartes claimed that it is “unwise to trust completely those who have deceived us even once”; he concluded that we cannot be completely certain that what we ‘see’ is real. This was his argument:

Our senses have deceived us in the past. It is unwise to trust completely those who have deceived us even once Therefore: One should not place absolute confidence in one’s senses.

You might reject the second premise of this argument, on the grounds that – while it is true that our senses have deceived us in the past – they have only done so in rather special conditions. For example, vision is unreliable when the objects being inspected are far away, or when we are tired, or not paying attention, or when the light is bad. Perhaps it is only in special cases like these that we should not place absolute confidence in our senses. In good conditions, perhaps you can be completely certain of what you see. For example, suppose you’re wide awake and in good lighting, and you are looking at a banana on a table right in front of you. Can’t you be certain that there is a banana before you – or at least, that there is something yellow and crescent shaped there? {Side-heading: dreams and hallucinations} But Descartes pointed out that error is possible even in what seem to be perfect conditions. Sometimes we have dreams that are so vivid that they perfectly mimic our waking experiences. Even in the best conditions – when you are fully awake, when the lighting is good, and so on – you can’t be completely certain that you are not dreaming. And so, Descartes argued, you can’t be completely certain of what you’re seeing.

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To put it rather more formally:

First Premise: You can never be completely certain that you are not asleep, having a vivid dream. Second Premise: You can only be completely certain that your perceptions are accurate if you can be completely certain that you are not currently asleep, having a vivid dream. Conclusion: You can never be completely certain that your perceptions are accurate.

We can reinforce Descartes’ point by thinking about hallucinations. The neurologist Oliver Sacks has described a strange experience he had after taking the drug Artane:

I was in the kitchen, putting on a kettle for tea, when I heard a knocking at my front door. It was my friends Jim and Kathy; they would often drop round on a Sunday morning. “Come in, door’s open,” I called out, and as they settled themselves in the living room, I asked, “How do you like your eggs?” Jim liked them sunny side up, he said. Kathy preferred them over easy. We chatted away while I sizzled their ham and eggs – there were low swinging doors between the kitchen and the living room, so we could hear each other easily. Then, five minutes later, I shouted, “Everything's ready,” put their ham and eggs on a tray, walked into the living room – and found it completely empty. No Jim, no Kathy, no sign that they had ever been there. I was so staggered I almost dropped the tray. (Oliver Sacks, Hallucinations, ch. 6.)

The story shows that even in what seem to be perfect conditions, perception can mislead. (Though the tale has a happy ending – Sacks had three portions of ham and eggs to himself). Now you may have good reason for being confident that you are not now hallucinating. Perhaps you have never hallucinated before, and have never knowingly consumed a hallucinogen, and have no history of psychiatric illness. Even so, Descartes argued, it is never beyond all doubt that one is hallucinating:

First Premise: You can never be completely certain that you are not currently hallucinating. Second Premise: You can only be completely certain that your perceptions are accurate if

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you can be completely certain that you are not currently hallucinating. Conclusion: You can never be completely certain that your perceptions are accurate.

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Descartes then raised the suggestion that certainty can be found in mathematics. Consider for example the claim that the sum of seven and five is twelve, or the claim that there is no largest prime number. Perhaps we can be completely certain that these claims are true. After all, these claims can be proven mathematically, in an a priori way – that is, without appeal to sense perception. {Side-heading: the evil demon} At this point, Descartes raises possibly the most famous thought experiment in the whole history of philosophy. We can paraphrase Descartes’ thought experiment as follows:

Suppose there exists an “evil demon” with supernatural powers. The demon is able to manipulate your mind to deceive you: he can induce hallucinations and he can modify your memories in whatever way he chooses. None of your present experiences are accurate: they’re all hallucinations caused by the demon. All of your real memories have been deleted, and replaced by false memories. You’re wrong about what your name is, wrong about where you grew up, wrong about what your body looks like, and so on. The demon is also able to scramble your thought processes, so that, for example, your mathematical calculations go wrong. If you try to find the sum 123 and 456, the demon can ensure that you get the wrong answer.

Descartes is not claiming that this story is plausible. As with the story about Notre Dame Cathedral, the point is that (pending some further argument) you can’t be completely certain the story about the evil demon is false. And so you can’t be completely certain of what your name is, or of where you grew up – you can’t even be completely certain the sum of 123 and 456 is 579. 3. The second meditation: finding certainty {Side-heading: I think, therefore I exist} At this point, you might be starting to think that Descartes’ search for complete certainty is hopeless. Seeking complete certainty, you might think, is like searching for a perpetual motion machine, a round square, or the fountain of youth: failure is guaranteed. But despite all of the sceptical points discussed in the last section, Descartes thought that complete certainty is possible: he claimed to be completely one-hundred-percent certain of his own existence. As he famously put it: I think, therefore I exist.

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You might object that Descartes can’t be totally sure that he exists, because he has no way of ruling out with certainty the hypothesis that his life is just an extended dream or hallucination. But Descartes had a clever reply to this objection: he pointed out that even if his life is a dream or hallucination, he must nevertheless exist in order to have the dream or the hallucination. Non-existent things can’t dream, and they can’t hallucinate. What about the evil demon? Can Descartes rule out the hypothesis that he believes that he exists only because this belief has been inserted by a powerful, deceitful demon? Descartes pointed out that an evil demon, however powerful, can only deceive existing things – it’s impossible to deceive something that doesn’t exist. So even if Descartes is being tricked by a demon, it’s nevertheless true that he exists. {Side-heading: I am a thinking thing.} Additionally, Descartes claimed to know with complete certainty something about what kind of thing he was: he could be completely sure that he was a ‘thinking thing’. (It sounds better in Latin: Descartes was completely certain that he was a res cogitans). Descartes used the term “thinking” very broadly, so that it covers all conscious mental activity, including:

• dreaming; • feeling pain; • seeing yellow; • doing a calculation; • having an urge to eat chocolate; and • deciding to sing a song;

Again, you might object that Descartes’ “thinking” is all part of a hallucination or dream – but Descartes can reply that hallucinating and dreaming are themselves sorts of thinking (in his broad sense of “thinking”).

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{Side-heading: Knowing one’s own mind.} Though this is slightly controversial, Descartes is often thought to have claimed that you can be completely certain about some of your own conscious mental states. To understand this claim, imagine that you go to the doctor complaining that you are in horrible pain. The doctor takes a blood sample and carries out some tests. Then she says:

We’ve identified your illness. It turns out that you’re not in pain after all: that’s just an illusion caused by your disease. It may seem to you that you’re in pain, but you’re not really in pain.

The doctor’s claim seems ridiculous: if it seems to you that you’re in pain, then you are in pain. You may not know what causes the pain, but there’s no doubt that you are in pain. You can be completely certain that you are in pain, or so it seems. Here’s another example. Suppose that you are looking a red piece of paper. Having read Descartes, you may find yourself doubting that there is really a red piece of paper in front of you: perhaps you are dreaming or hallucinating. Even so, it seems that you can be certain that you are having the experience of red. The cause of this experience may be obscure to you, but the experience itself cannot be doubted. So there is some plausibility to the claim, which has been attributed to Descartes, that one can know some things about one’s own mental states with complete certainty. It would be a mistake to claim that we all have complete and perfect knowledge of our own conscious mental states. It’s possible to misremember one’s mental states. For example, you might “remember” having a headache yesterday when in fact you haven’t had a headache for months. Arguably, it’s also possible to fail to notice one’s conscious mental states. For example, a marathon runner might, in her determination to win a race, fail to notice an ache in her ankle. Nonetheless, it is plausible to think that we can know a great deal about our current mental states with the sort of complete certainty that Descartes sought.

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4. The third meditation: clear and vivid perceptions {Side-heading: Taking Stock} At the end of the second meditation, the good news is that Descartes seems to have shown that seeking certainty is not like seeking the fountain of youth: there really are some truths that can be known with certainty. The bad news is that so far Descartes has only managed to identify very few such truths: Descartes can be certain that he exists; he can be certain that he is a thinking thing; and perhaps he can be certain about the character of his own current mental states. {Side-heading: vivid and clear perception} Descartes claimed that these truths that are known with certainty have something in common: he knew them to be true by what he called “vivid and clear perception” (or, in some translations, “clear and distinct perception”). He then suggested a more general rule:

Whenever I vividly and clearly perceive that something is true, I am entitled to believe with complete certainty that it is indeed true.

This claim (let’s call it “the V&C Rule”) was central to Descartes’ thought – but it’s more than a little obscure. What is “vivid and clear perception”? It’s important to stress that Descartes is not talking about sense perception here. As we’ve already seen, Descartes denied that we can achieve complete certainty using sense perception. What Descartes had in mind was this. When he considered the claim I exist, Descartes couldn’t help believing it – he felt compelled to believe that he existed. It is this compulsion that Descartes called ‘vivid and clear perception’. Here’s an example. Think about the statement that five is the sum of two and three. When you think about this statement, you are likely to find that you can’t help believing that the statement is true – you feel forced to believe that the sum of two and three is five. This inner duress is what Descartes called a “vivid and clear perception”. So the V&C rule implies that you are entitled to believe with complete certainty that the sum of two and three is five. What’s more, Descartes thought that by inspecting a rigorous mathematical proof of a theorem, you can achieve vivid and clear perception of the truth of the theorem. So Descartes thought that the V&C rule guarantees that the theorems of mathematics are all true.

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{Side-heading: the evil demon, again.} Now Descartes had already considered the claim that simple mathematical truths can be known with certainty. There, he presented a compelling objection to this claim. He pointed out that it is difficult to rule out with certainty the hypothesis that our mathematical calculations are being scrambled by an evil demon. This point can be used to formulate an objection to the V&C Rule. It would seem that a powerful malicious demon might be able to give you a powerful compulsion to believe that, say, that the sum of two and three is seven or that frogs can speak ancient Greek. It’s not clear that Descartes can rule out with certainty the hypothesis that his “vivid and clear perceptions” have been implanted in him by an evil demon, and are misleading. {Side-heading: Descartes’ solution} Descartes responded to the evil demon problem by arguing that God would not allow a demon to trick him in this way. God is both omnipotent and perfectly good, and so He must ensure that His people are not deceived by evil demons in this horrible way. Descartes’ argument might be reconstructed as follows:

First Premise: God exists. Second Premise: If God exists, He would ensure that everything that I, Descartes, vividly and clearly perceive is true. Conclusion: Everything that I, Descartes, vividly and clearly perceive is true.

This is a powerful argument, but it leaves Descartes with the challenge of establishing its first premise: that is, Descartes must now argue that God exists. As we saw in chapter one, this is not easy. {Side-heading: Descartes’ attempt to prove that God exists} Descartes gave two arguments for the existence of God: one in the third meditation, another in the fifth. We’ll take a look at the latter argument:

(1) I vividly and clearly perceive that God has all perfections. (Premise) (2) Whenever I vividly and clearly perceive that something is true, I am entitled to believe with complete certainty that it is indeed true. (Premise) (3) God has all perfections. (From 1 and 2) (4) Existence is a perfection. (Premise) (5) God exists. (From 3 and 4)

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If you’ve read chapter one, you might notice that this is a simplified version of Anselm’s ontological argument. The argument begins with a report about what Descartes vividly and clearly perceived: he vividly and clearly perceived that God has all perfections. Given the V&C rule – which is premise (2) of the argument – it follows that God really does have all perfections. Premise four is the claim that existence is a perfection: for discussion of this claim, see chapter one. The conclusion is then that God has existence, or to put it in more natural English, that He exists. {Side-heading: The Cartesian circle} We’ve already discussed the question of whether the ontological argument is sound, and we don’t to repeat ourselves by addressing the question again now. Instead, we want to look at a deeper problem with the strategy that Descartes employs here. Descartes sought to demonstrate that the V&C rule is true by appeal to theism. However, his argument for theism itself has the V&C rule as a premise! And so it seems that Descartes’ argument runs in a circle:

Philosophers sometimes call this the “Cartesian circle”. Many philosophers believe that Descartes’ attempt to refute scepticism failed precisely because his argument was circular.

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To see the problem, think again about the evil demon problem. According to the story, there exists a very powerful demon who sets out to uses his powers to deceive Descartes. He causes Descartes to hallucinate, and he inserts false memories into Descartes’ mind. Perhaps worst of all, the demon manipulates Descartes’ reasoning – for example, the demon messes up Descartes’ mathematical calculations. Until Descartes can show that this story is false, just about everything he used to believe is under suspicion. His memories and his senses can’t be trusted. Even his beliefs about simple arithmetic (e.g. five is the sum of two and three) may be mistaken. To deal with this concern, Descartes gave his proof that God exists; he then inferred that the story about the evil demon is false, because God (who is benevolent and all-powerful) would not allow a demon to manipulate Descartes in this grotesque way. Even setting aside objections to the details of Descartes’ supposed proof, the strategy is problematic. For it seems that Descartes has no way of showing that, while constructing his proof, his reasoning is not being scrambled by an evil demon. Descartes will be in a position to trust his proof of the existence of God only when he has established that he is not subject to demonic manipulation, but he can only establish that he is not subject to demonic manipulation if he can first show that God exists. {Side-heading: Did Descartes fail?} Many commentators believe that Descartes’ attempt to deal with his sceptical doubts failed, precisely because his reasoning was circular. However, a number of enthusiasts for Descartes’ work offer interpretations of his argument which avoid circularity.1 5. Descartes’ Legacy {Side-heading: Descartes, the first modern philosopher} Descartes’ Meditations is often said to be the founding work of modern (as opposed to medieval) philosophy. This is so despite the fact that some of the arguments in Descartes’ book (for example, his supposed proofs of the existence of God) look a lot like the arguments of the medieval philosophers who preceded him. In this final section of the chapter, we look at two ways in which Descartes’ work influenced later philosophy.

1 Bernard Williams is one commentator who has argued that the charge of circularity is mistaken. See his Descartes: The Project of Pure Enquiry.

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{Side-heading: Descartes’ influence} Anyone who’s heard a past event described by several independent eye witnesses will know that memory and perception are fallible. Anyone who’s practiced long division knows that it can be easy to make mistakes in reasoning. But Descartes bought to the forefront of philosophical discussion the fear that these faculties may mislead us radically and systematically: perhaps all of our sense perceptions are hallucinations; perhaps all of our memories are inserted by a malicious demon; perhaps all of our reasoning is muddled by chronic cognitive disorder; and so on. Since Descartes, it has often been thought that a central task of philosophy is that of rebutting these “sceptical” worries, and establishing that we are justified in taking our perceptions and memories at face value, and in trusting the conclusions of our reasoning. While they have worried a great deal about memory, sense perception and reasoning, philosophers influenced by Descartes have often felt that our knowledge of our current conscious experience is secure: when you are in pain, or cold, or having the experience of yellow, you know it. So after Descartes, it has often been an aspiration of philosophers to show that we can achieve knowledge of “external” things (i.e. things outside one’s own current conscious experience) on the basis of one’s knowledge of “internal” things (i.e. one’s current conscious experiences). This Cartesian way of thinking about the goals of philosophy has its critics, of course. The philosopher Richard Rorty once said that when confronted by sceptical questions such as “How do you know that you’re not the victim of an evil demon?” the correct response is simply to tell the sceptic to “get lost”. What’s more, since the late nineteenth century the trend in philosophy and psychology has been to emphasize the fact that many of our mental processes are unconscious. So even if it’s true that you can sometimes know for certain that you are in pain or that you are hungry, these are exceptional cases: much of what happens in the mind cannot be discovered by “looking inwards”. Questions (1) The following statements are all generally believed to be true. In your opinion, which of them are certainly true? Why? (a) 2+2=4. (b) France is in Europe. (c) There are no married bachelors. (d) E=mc2. (e) At least one thing exists. (f) Not every statement is true. (g) You exist.

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(2) In section 3, we claimed that non-existent things can’t dream and can’t have hallucinations. But is this right? Here’s an objection: In Shakespeare’s play, Hamlet has a hallucination. Hamlet doesn’t exist. Therefore: Non-existent things can have hallucinations. What do you think of this argument? (3) Philosophers use the word ‘introspection’ for the process of ‘looking inwards’ to find out about what’s going in your mind. Is it always possible by introspection to find out whether you are… … (a) in pain? … (b) in love? If you gave different answers to questions (a) and (b), why? Why are pain and love different in this respect? (4) (a) Can you think of any circumstances in which you might mislead a child for his or her own benefit? (b) Assuming that God exists, could there be circumstances in which God would want people to have false beliefs? (c) Was Descartes right to think that, if God exists, whatever he vividly and clearly perceived is true?

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