Skepticism, First Encounter

The previous article ended with a worry deflected rather than answered. Pressed properly, the worry produces an argument that almost nobody believes the conclusion of, and that almost nobody can say what is wrong with. That combination is what makes it worth studying.

The position

Two clarifications prevent most of the misunderstandings.

Skepticism in this sense is not doubt. The skeptic need not claim to be unsure whether there is a floor, and philosophical skeptics have generally walked around buildings rather than through walls. The claim is about the epistemic standing of ordinary beliefs, not about anyone's confidence in them.

And it is not the same as the everyday use of the word, where a skeptic is someone who demands evidence before accepting a claim. That person is applying a standard. The philosophical skeptic argues that the standard cannot be met, and often that it cannot be met by anything.

Descartes's method

The method appears in the First Meditation, and its purpose was constructive. Descartes wanted a secure starting point for the sciences, and his strategy was to demolish everything demolishable and build from whatever was left. The doubts are instruments, and treating him as a skeptic reverses his intention.

He proceeds by attacking classes of belief rather than beliefs one at a time, since there are too many to check individually. First the senses, which are known to mislead about small and distant things. That much is admitted by everyone, and it does not touch the belief that one is sitting by a fire holding a piece of paper.

The argument's force comes from a fact about dreams rather than from a speculation. Descartes points out that he has been convinced, while asleep, of exactly the things he is now convinced of, and the conviction felt the same. In the Cottingham, Stoothoff, and Murdoch translation of the First Meditation: "Indeed! As if I did not remember other occasions when I have been tricked by exactly similar thoughts while asleep!"

There is no experiential test that distinguishes the cases, because any test would be an experience, and any experience can be dreamt. Pinching yourself is exactly the sort of thing that happens in dreams.

Descartes goes further still. A dreamer at least has waking experience to build from, so perhaps the simplest components of experience are secure. To close that gap he supposes a deceiver of the utmost power devoted to misleading him, in which case even the appearance of simplicity guarantees nothing.

Hilary Putnam introduced the case in this form, and it is an improvement on the demon in one respect. It requires no supernatural agent, only technology of a kind nobody can show to be impossible, which makes it harder to dismiss as idle.

The argument itself

The hypotheses are only the setup. What does the damage is an entirely ordinary-looking principle about what follows from what.

The first premise is an instance of something very hard to give up: that justification carries across an entailment you recognise. Having hands entails not being a handless brain in a vat, and you know it does. If you could be justified in the first while being unable to rule out the denial of the second, then justification would fail to transmit across obvious consequences, which seems to make it useless.

The second premise is what the hypotheses were constructed to secure. The scenario is designed so that all your evidence is identical either way. Nothing you could look at, remember, be told, or work out discriminates the cases, because the scenario stipulates that the experiences are the same.

And the conclusion generalises. Nothing about hands was special, so the same argument runs against every belief about the world outside your own mind.

The shape of the responses

Four broad strategies are on offer, and each denies a different part of the argument.

Deny the second premise. G. E. Moore argued that the existence of his hands is better supported than any premise in an argument to the contrary, so the sensible move is to run the argument backwards: I have hands, so I am not a handless brain in a vat. The response is often felt to beg the question, and it makes a fair point about comparative certainty.

Deny the first premise. Justification might attach only relative to the alternatives that are relevant in a context, so that ruling out the vat is not required for ordinary knowledge. This is the most-developed line in the contemporary literature, and it has to explain why some alternatives count and others do not without simply stipulating the answer.

Attack the hypothesis. Putnam argued that a brain in a vat could not truthfully think that it is a brain in a vat, because what a word refers to depends on causal contact with the things it refers to, and a lifelong envatted brain has never been in contact with brains or vats. If the argument works, the hypothesis cannot be coherently entertained by the person it is supposed to worry.

Accept the conclusion and separate it from practice. Nothing in ordinary life changes if the argument is sound, since the beliefs remain as useful, as well supported by each other, and as unavoidable as before. The cost is admitting that the everyday use of "know" is systematically false, which is a large thing to admit for the sake of preserving an argument.

Why it stays on the syllabus

The value of the argument is not that anyone is persuaded. It is that every theory of knowledge can be tested against it, and what a theory says about the vat reveals what it thinks justification is for. That is why the intermediate tier returns to it once the structural and modal theories are available.

The next article assembles the analysis of knowledge that all of this has been circling.