The sceptical arguments

The sceptical arguments have a bad reputation, mostly earned by the way they are usually presented. Somebody says you cannot prove you are not dreaming, everybody agrees that is interesting, and the conversation moves on without anyone having stated an argument. This lesson states three, at full strength, and does not answer any of them. The answering is lesson four.

Take the standard from lesson one seriously while reading. These arguments are all valid. If you reject the conclusion, you owe a premise.

The dream argument

Descartes opens the Meditations on First Philosophy (1641) with it, though the observation is much older and appears in Zhuangzi in the fourth century BC, where a man dreams he is a butterfly and wakes unsure which way round it goes.

Descartes' version is tighter than the folklore. He does not claim you might be dreaming right now and leave it there. He argues:

1. There is no mark by which waking experience can be told from dreaming experience with certainty.
2. If there is no such mark, then any belief you hold on the basis of experience might be a belief formed while dreaming.
3. A belief formed while dreaming, about the world outside you, is not knowledge.
4. So no belief you hold on the basis of experience is knowledge.

Premise 1 is the one people attack, usually by saying dreams are incoherent, or that you can read text in waking life and not in dreams, or that pinching works. These replies share a defect: each cites a feature of dreams noticed while awake. To use the feature you would have to know you were awake when you noticed it, which is what is in question. The reply is not obviously wrong, but it is circular in a way that needs argument rather than assertion.

The more effective objection runs at premise 3, and it is worth seeing now because the same move works elsewhere. Dreaming is parasitic. You dream of houses and horses and your mother, and those concepts came from somewhere. A mind that had only ever dreamed would have nothing to dream about. So the dream argument, pushed all the way, may undercut itself: universal dreaming is not a coherent hypothesis. This does not save any particular belief, since tonight's dream can still be wrong about everything, but it limits the damage.

Example. Someone says: "I know I am awake because I just did a reality check. I looked at my hand, looked away, looked back, and it was the same." What premise is this aimed at, and does it hit?

It is aimed at premise 1, offering stability of perception as the mark that distinguishes waking from dreaming. It does not hit, for the circularity reason above: the claim that hands are unstable in dreams is something learned while apparently awake, and lucid dreamers report perfectly stable hands. But notice the shape of the failure, because it recurs throughout the course. The test is not useless, it is just useless against this argument, since the argument has already put in question the very body of experience the test appeals to. A test that assumes what is in dispute cannot settle the dispute.

Now you. A second person says: "The dream argument proves too much. If I cannot tell dreaming from waking, then I cannot tell whether I am dreaming the argument, so I have no reason to accept its conclusion either." Is that a good objection?

The evil demon

Descartes escalates in the same work. Suppose not a natural lapse of judgement but a deceiver of the utmost power and cunning, who devotes all his energy to misleading you. The sky, the earth, your body, and the memory of yesterday are all produced in you by the demon. What survives?

Very little, and the little that survives is the famous part. You cannot be deceived about your own existence while being deceived, since deception requires a subject. Cogito, ergo sum is not a syllogism so much as a performance: the act of doubting establishes the doubter. It is worth being precise about what it delivers, because it is routinely oversold. It gives you a thinking thing, existing now. It does not give you a body, a past, a world, or the identity of the thinker with the person whose name you sign.

It is also worth noticing what the demon hypothesis costs its user. A deceiver of unlimited power and unlimited malice is a large thing to posit, and lesson nine asks what a hypothesis has to do to earn its keep. Descartes does not need the demon to be plausible, only possible, but that concession is doing real work and a reader should mark it now: the sceptic is running on bare possibility, and the strongest reply in lesson four is that bare possibility is cheap.

The demon is stronger than the dream in two ways. It reaches memory, so the argument about dreaming being parasitic on waking experience no longer applies: the demon could have manufactured the concepts. And it reaches reasoning, since a sufficiently powerful deceiver could make invalid inferences feel valid. That second reach is what makes the demon genuinely alarming rather than merely striking.

Example. A student says the cogito proves too little to be interesting, since knowing that something is thinking gets you nowhere. Is that fair?

Half fair, and the half that is wrong matters. It is true that the foundation is very thin, and the attempt to build outward from it, via an argument that a perfect God would not permit systematic deception, is the least admired part of the Meditations. But the cogito is not really a premise to reason from, it is an existence proof: it shows the sceptical argument has a floor, that not everything can be doubted at once. That matters for the shape of the debate, because a scepticism with no fixed point consumes itself, while one with a fixed point is a position that can be argued with. The student is right that the floor is low and wrong that a floor is worthless.

Now you. Both the dream and the demon conclude that you cannot trust your experience. Why is the demon the stronger argument?

The brain in a vat

The modern restatement, from Hilary Putnam in Reason, Truth and History (1981), removes the theology. You are a brain in a vat of nutrients, wired to a computer that feeds you exactly the experiences you are having now. Everything seems as it does. Nothing you could check would come out differently.

Its usual form is not the four-step argument above but a shorter and sharper one built on a principle from logic:

1. You do not know you are not a brain in a vat.
2. If you know you have hands, and you know that having hands entails not being a brain in a vat, then you know you are not a brain in a vat.
3. So you do not know you have hands.

Step 2 is an application of closure: knowledge is closed under known entailment, meaning that if you know p, and you know p entails q, then you are in a position to know q. Closure looks almost undeniable. It is what licenses ordinary reasoning: knowing the train leaves at six and knowing that entails it does not leave at seven, you know it does not leave at seven. Denying it has a cost that lesson four takes seriously.

Premise 1 looks undeniable too, since the vat scenario is constructed to be indistinguishable from the inside. So the argument is valid, both premises are plausible, and the conclusion is that you do not know the most obvious thing about yourself. That is the sceptical problem in its cleanest modern form, and it is why the vat rather than the demon is what contemporary epistemology argues about.

Putnam himself thought the hypothesis self-refuting, by an argument about what words refer to: a brain that had only ever been in a vat could not mean by "vat" what we mean, since its word never contacted any vats, so its sentence "I am a brain in a vat" could not be true. The argument is ingenious and contested, and it only touches lifelong vat-dwellers, not someone envatted last Tuesday.

Example. Someone objects: "The vat story is unscientific. We have no such computers, no such surgery, and no reason to think anyone is doing this." Does the objection work?

No, and seeing why is important. The sceptic is not claiming the scenario is likely, or actual, or even physically achievable. The argument needs only that it is possible and that you cannot rule it out from the inside. Premise 1 says you do not know it is false, and improbability is not knowledge of falsehood: you do not know you will not be struck by lightning today, though it is very unlikely. The objection would matter if the argument were an inference to the best explanation, which is exactly the form lesson four uses against it. Here it misses.

Now you. Compare the three arguments. Which of the three conditions from lesson one does each attack, and do they all attack the same one?

Why these are not silly

Two features make the sceptical arguments worth a lesson rather than a joke.

First, nobody has produced an answer that all sides accept, and the attempts are the history of modern epistemology. Descartes tried to argue from a non-deceiving God, which most readers find circular. Kant tried to argue that the very possibility of experience presupposes an objective world. Moore held up his hands. Putnam used the theory of reference. Every one of these is a serious position with serious critics, and lesson four takes the best of them.

Second, the arguments are not idle. Their structure is reused wherever someone claims a hypothesis cannot be ruled out and concludes that you therefore know nothing about the matter, which is a move made constantly in arguments about God, about consciousness, and about whether the universe is fine-tuned. Recognising the shape here makes it recognisable later, and lesson nine gives the criterion for when it is legitimate.

What the arguments share is that they trade on a gap between experience and its causes. You have the experiences. The claim that a world outside causes them is an inference, and lesson four's most promising reply argues that it is a very good inference, which requires the machinery of lesson five. Before that, the version of this argument that is currently taken most seriously, and most often misreported, is the one where the deceiver is not a demon but ourselves.