External world

6

How Can We Know about the External

World?

You know that the earth is round, that penguins inhabit Antarctica, that trees shed their leaves in the fall, that you have a heart, and so on and so on. In other words, you know a lot about the “external world,” including your own body. That much is obvious.1

Or is it? Consider the hypothesis that your entire life has been a remarkably vivid dream. Not only have you been dreaming the whole time, but the earth never existed. No penguins, trees, nothing like that. In fact, you don’t even have a heart. You are a heartless android, lying comatose in a robot junkyard on a planet orbiting the star Kepler-11. “From the inside,” things seem exactly the same to you: you seem to see this page, you seem to remember that penguins inhabit Antarctica, and so on, even though there is no page, and no Antarctica. So how can you know that this “android hypothesis” is false? That question can seem very difficult to answer, which suggests that you can’t know that the android hypothesis is false.

This claim of ignorance might not seem so bad by itself, but once it is conceded, it is difficult to stop ignorance from spreading much more widely. Take, for example, one thing that you apparently know; namely, that penguins inhabit Antarctica. Now the claim that penguins inhabit Antarctica straightforwardly entails that the android hypothesis is false. If penguins inhabit Antarctica, you are not a dreaming android who lives in a penguin-free world. So, if you know that penguins inhabit Antarctica, you can perform an elementary logical inference and come to know that the android hypothesis is false. So, if you can’t know that the android hypothesis is false, you don’t know that penguins inhabit Antarctica. By the same argument, neither do you know that the earth is round, that trees shed their leaves in the fall, and so on. In short, if you can’t know that the android hypothesis is false, you are completely ignorant about the external world; that is, external world skepticism is true. (A skeptic about some subject matter M is someone who denies that we have knowledge about M.)

Still, you might wonder whether even external world skepticism is worth worrying about. Suppose you’re offered a choice between going on a roller coaster ride and entering the roller coaster simulator. The simulator is perfect: as far as excitement goes, it’s just as good as the real thing, although you aren’t really rattling down a narrow track at 100 mph. The choice doesn’t seem to matter much (actually, you might even prefer the simulator on the grounds that it’s much safer). Here, virtual reality is no worse than reality itself. Isn’t that true in general? Why care whether you’re a dreaming android? The thrills and spills of life would be the same in any case.

But this reaction is overly complacent. The dreaming android has no friends, has no mother who loves it, and has never accomplished anything—vividly dreaming that you are acing your final exams is not a way of doing well in school. Having friends, to say nothing of a mother who loves you, is a valuable thing. (Imagine discovering that someone whom you thought a faithful friend was just pretending.) So if you are a dreaming android, you are in a very unfortunate predicament—friendless, unloved, and unaccomplished. You should want to be reassured that you are not in this predicament. That is, you should want to know that you have friends, are loved, and so forth. If external world skepticism is true, reassurance that your life is not an empty sham is forever beyond your reach.

The readings in this chapter respond to the threat of skepticism about the external world. (One exception, as we will see below, is the essay by Rae Langton.) Before getting to the many different responses, it will help to set out the argument for the skeptical conclusion more precisely. And in the course of doing that, we will see how the argument is a particular instance of a general form of skeptical argument.

1. If you know p, you can know that SH is false.

2. You can’t know that SH is false.

A General Skeptical Argument

Let a skeptical hypothesis be a hypothesis according to which the world is different from how you take it to be. We have already seen one skeptical hypothesis, according to which you’re a dreaming android and the earth never existed. There are other similar skeptical hypotheses, the most famous of which is René Descartes’s demon hypothesis: “some malicious demon of the utmost power and cunning has employed all his energies in order to deceive me.” The contemporary version of the demon hypothesis is the brain in a vat hypothesis: you are a brain kept alive in a vat by some evil scientist and stimulated so that “from the inside” things seem exactly as if you see this page, and so on.

These hypotheses are global skeptical hypotheses; that is, if they are true, almost nothing you take yourself to know about the external world is true. But skeptical hypotheses can be more modest. Indeed, in an everyday situation in which you wonder whether you really are right to think that you left your laptop at home, you are entertaining a very modest skeptical hypothesis—that the world is very similar to the way you take it to be, except that your laptop is not at home. Philosophers have devised many other skeptical hypotheses that are intermediate in strength between global skeptical hypotheses and very modest skeptical hypotheses like the one just mentioned.

For instance, there is the no other minds hypothesis, according to which you are the only creature with a mind—everyone else behaves just as if they believe, feel, and perceive but are actually entirely mindless (see Saul Kripke’s “Wittgenstein and Other Minds (chapter5-4.xhtml)” in Chapter 5 (chapter5.xhtml)). And there is the unexpected future hypothesis, according to which the future is radically different from the past—if this hypothesis is true, bread will not nourish tomorrow, the sun will not rise tomorrow, and so on (see the introduction to Chapter 4, “How Can We Know about What We Have Not Observed? (chapter4.xhtml)”).

Now take a skeptical hypothesis SH, and any claim p that entails that SH is false. We can argue that you don’t know p as follows:

3. You don’t know p.

So:

For example, suppose you think ( p) that your bike is where you left it, padlocked to a bike rack. Let SH be the modest skeptical hypothesis that your bike has been stolen. The claim p (your bike is where you left it) entails that SH (your bike has been stolen) is false, in other words that your bike has not been stolen. So we can argue that you don’t know that your bike is where you left it as follows:

1†. If you know that your bike is where you left it, you can know that your bike has not been stolen.

2†. You can’t know that your bike has not been stolen.

So:

3†. You don’t know that your bike is where you left it.

This form of argument—If P then Q, it is not the case that Q, so it is not the case that P—is called modus tollens.

Let us look more carefully at premise 1. Suppose you know that all fish have gills and that Wanda is a fish. Now the statement that all fish have gills and Wanda is a fish entails that Wanda has gills. So you are now in a position to draw the conclusion that Wanda has gills from what you already know. And if you go ahead and do that, it seems very plausible that you will end up knowing that Wanda has gills. In general, one way to extend our knowledge is to trace out the logical consequences of what we already know: this happens whenever someone proves a theorem in mathematics, for example. Put more precisely: if p entails (or logically implies) q, and you know p, then you are in a position to know q. This is (one version of ) a principle called closure.

Given closure, premise 1 of the skeptical argument is true. Closure is difficult to deny and the argument is valid, so when faced with a skeptical argument of this form you have two options: deny premise 2 or accept the conclusion.2

We can generate an argument for external world skepticism by letting SH be a global skeptical hypothesis, say, Descartes’s demon hypothesis, and letting p be any claim about the external world that entails that the demon hypothesis is false, say, that the earth is round:

1*. If you know that the earth is round, you can know that the demon hypothesis is false.

2*. You can’t know that the demon hypothesis is false.

So:

3*. You don’t know that the earth is round.

Again, assuming closure, there are two options: deny premise 2* or accept 3*, the skeptical conclusion.

Responses to External World Skepticism

The readings from David Hume, G. E. Moore, and Jonathan Vogel offer contrasting responses to external world skepticism.3

According to Hume, our senses provide scant evidence for hypotheses about the external world. In a paragraph omitted from the selection he writes: “ ’Tis impossible . . . that from the existence or any of the qualities of [perceptions], we can ever form any conclusion concerning the existence of [objects].” So Hume is a (rare) example of a real-life skeptic: he accepts the conclusion of the skeptical argument. His main concern is not so much to defend skepticism (which he thinks is pretty much unassailable) but rather to give a psychological explanation for why we think that there is an external world of familiar tables, chairs, penguins, and so on, even though we have no good reason for doing so.

Vogel, in effect, directly replies to Hume. While Hume thinks that our senses cannot show us that we are not brains in vats or deceived by an evil demon, Vogel thinks otherwise, and so denies the second premise. Vogel argues that the “real world hypothesis”—that the earth is round, you have a head, are reading this book, and so on—provides a much better explanation of your “sensory experiences” (or, in Hume’s terminology, “perceptions”) than any global skeptical hypothesis. So you have a good reason to believe the real world hypothesis by an “inference to the best explanation.”4

Moore is principally concerned to deny the conclusion of the skeptical argument, rather than to explain which premise is false. He tries to turn the tables on the skeptic by offering what he claims is a proof of the existence of things like tables and books. For example: here is a book (Moore holds up a copy of this book), here is another book (Moore holds up a copy of his own famous book on ethics, Principia Ethica), therefore books exist. Of course, the skeptic will not grant that this is a proof, on the grounds that Moore does not know the premises. But, as Moore points out, in ordinary life we take arguments of this sort “as absolutely conclusive proofs of certain conclusions.” For instance, we allow that someone can prove that there are at least three misprints on a page from the premises “There’s one misprint here, another there, and another here.” And if we really can prove such things, we must have knowledge about the external world. Why isn’t the skeptic just being unreasonable in rejecting Moore’s proof ?

Kantian Skepticism

Rae Langton’s essay defends a limited but nonetheless fascinating form of skepticism, which she finds suggested by the work of the German philosopher Immanuel Kant (1724–1804). This Kantian skepticism (or “Kantian humility,” as Langton calls it) is not external world skepticism. As both Moore and Langton note, Kant thought we had plenty of knowledge about the external world.

However, in his Critique of Pure Reason, Kant argues for another kind of skepticism: as he put it, we have no knowledge of “things in themselves.” What did he mean by that? Langton offers an answer, and a defense of a kind of skepticism that she thinks is at least in the spirit of Kant’s actual view. According to Langton, the physical sciences can only penetrate so far into reality: there is a layer further down that is in principle beyond their reach. If that is right, then although we can know that there are books, and that we have friends, ignorance of the fundamental nature of the world is part of the human condition.