The Big Picture
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hooked directly into our neurons rather than the real outside world. We
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could be living in a computer simulation like in The Matrix, and the true
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external reality could be something very different than we suppose. Finally,
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as his critics have pointed out, Descartes shouldn’t only worry that he is
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dreaming; he should also worry that he is being dreamed. (In the Hindu
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Vedanta tradition, all the world is a dream of Brahma.)
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In 1857, naturalist Philip Henry Gosse published a book, Omphalos, in
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which he attempted to reconcile the age of the Earth as inferred from geo-
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logical evidence (very old) with that inferred from the evidence of the Bible
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(very young). His idea was simple: God had created the world a few
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thousand years ago, but with all the signs of being much older, including
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mountain ranges that would take millions of years to form, and fossils of
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apparently great antiquity. Gosse’s title came from the Greek word for “na-
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vel,” since part of his inspiration was that the first human, Adam, must have
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been a complete person, and therefore had a navel, even though no woman
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had given birth to him. Versions of his idea are promoted to this day by
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some Christian and Jewish creationists, who use it to account for cosmo-
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logical evidence of light that left distant galaxies billions of years ago.
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It’s easy to see how the Omphalos hypothesis leads to yet another skepti-
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cal scenario, which has waggishly been labeled “Last Thursdayism”— the
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idea that the entire universe was created intact just last Thursday, complete
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with all of the records and artifacts that seem to point to the existence of
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an extended past. Bertrand Russell once pointed out that there’s no way of
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being completely sure that the world didn’t spring into existence five min-
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utes ago. You might think that this can’t be true, since you have clear mem-
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ories of last Wednesday. But a memory— just like a picture, or a diary— exists
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now. We take memories and records as (somewhat) reliable guides to the
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past, since that seems to have worked for us thus far. It’s logically possible,
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however, that all of those purported memories, as well as our impressions
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that they are reliable, were created along with everything else.
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Without really meaning to, physicists have been led to consider cosmologi-
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cal models that veer uncomfortably close to the Omphalos hypothesis. In
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the nineteenth century, Ludwig Boltzmann contemplated a universe that
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has lasted forever but has almost everywhere and almost always been in a
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state of uniform, uninteresting disorder. The individual atoms in such a
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universe would be in constant motion, randomly shuffling and bumping
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into one another. But eventually, if we wait long enough, the motions of the
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atoms will bring them just by chance into a highly ordered state— for ex-
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ample, much like the Milky Way galaxy, which astronomers of the time
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thought was the entire universe. (The ancient Roman poet Lucretius sug-
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gested a very similar picture; like Boltzmann, he was an atomist, trying to
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account for the origin of order in the world.) This configuration would
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evolve as normal, eventually dissipating back into the surrounding chaos
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as the universe reaches its ultimate heat death. At least until the next fluc-
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tuation.
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There is one quite significant problem with Boltzmann’s idea. Fluctua-
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tions from disorder to order are rare, and larger fluctuations are much more
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rare than smaller ones. So if Boltzmann had been right, there’s no need to
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wait for something as impressive and grand as the Milky Way, with hun-
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dreds of billions of stars, to shuffle its way into existence. It’s far easier for
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something smaller, like the sun and its planets, to emerge out of the chaos.
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And when you think about it, the vast majority of conscious, thinking crea-
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tures in this kind of universe will be single individuals who have fluctuated
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into existence all by themselves— just long enough to think, “Hm, I seem
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to be all alone in this universe,” and then die. Indeed, why even bother with
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an entire body? Most of these lonely souls will be the minimal possible
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amount of matter that could qualify as a thinking being: a disembodied
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brain, floating in space.
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For obvious reasons, this has become known as the “Boltzmann Brain”
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scenario. To be clear, nobody thinks the universe is actually like that. The
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problem is that it seems like it should be true, if the universe is infinitely old 18
and randomly fluctuating. In that case, the appearance of Boltzmann
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Brains seems inevitable. And since the overwhelming majority of observers
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in such a universe are disembodied brains, why am I not one?
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There is a way out of the Boltzmann Brain problem that is simple, but
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wrong. It’s to say “Maybe most observers in the universe are random fluc-
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tuations, but I’m not one, so I don’t really care.” How do you know you’re
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not a random fluctuation? You can’t say that you have memories of a long
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and fascinating life, since those memories could have fluctuated into exis-
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tence. You might point to your surroundings— there’s a room, and a win-
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dow, and outside seems to be an elaborate environment, all of which is
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much more than would be predicted by this crazy fluctuation scenario.
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And that’s true; most people in this crazy fluctuation scenario shouldn’t
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find themselves surrounded by rooms and neighborhoods and all the stuff
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we are pretty sure constitutes our local environments. But some of them
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bsp; will. If the universe is truly infinitely old, there will be an infinite number
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of such environments. And the overwhelming majority of them will have
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randomly fluctuated into existence directly from the surrounding chaos.
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You may think, for example, that you are reading a book by a person named
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Sean Carroll, who probably exists (or once did, depending on when you are
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reading). But given an infinite universe, it’s much easier for this book, with
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my name on the cover and picture on the flap, to randomly fluctuate into
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existence by itself than for this book and my actual person to fluctuate into
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existence. Even if we grant you the reality of what you seem to experience
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in your local environment, in Boltzmann’s cosmology you have no reason
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whatsoever to actually trust in the existence of anything else— including
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anything beyond your immediate perception, or anything you might think
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you are remembering about the past. All of your memories and impressions,
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with probability close to 1, just fluctuated into existence themselves. It’s the
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ultimate skeptical scenario.
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Are you sure you’re not a Boltzmann Brain? Or at least, do you know your
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local environment didn’t recently fluctuate into existence? How do you
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know you’re not a brain in a vat, or a character in some more advanced be-
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ing’s video game?
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You don’t. You can’t. If by “know” we mean “know with absolute, meta-
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physical certainty, without any conceivable possibility of being wrong,”
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then we cannot ever know that none of these scenarios is correct.
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Later in life, Wittgenstein himself contemplated a way out of this co-
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nundrum. In On Certainty, one of the first things he writes is “From its
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seeming to me— or to everyone— to be so, it doesn’t follow that it is so.” But 25
he immediately follows this with “What we can ask is whether it can make
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sense to doubt it.” Put conversely, something might conceivably be true, but
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there might not be any point in assigning much credence to it.
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Consider the most dramatic kinds of skeptical scenarios, like Descartes’s
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worry that all of his knowledge of the external world is unreliable because
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he is being fooled by an evil demon. We would like to prove that this is
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wrong, or at least collect some strong evidence against it. But we can’t. A
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sufficiently powerful and clever demon would be able to influence all of our
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appeals to logic and evidence. “I think, therefore I am”; “Existence is an
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attribute of perfection, therefore God exists”— these might very well seem
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logically sound to you (or at least to Descartes). But that’s just what the evil
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demon would want you to think! How can we be sure that the demon isn’t
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tricking us into logical fallacies?
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Any of the various skeptical scenarios about the existence of external
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reality, and our knowledge thereof, might very well be true. But at the same
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time, that doesn’t mean we should attach high credence to them. The prob-
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lem is that it is never useful to believe them. That’s what Wittgenstein
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means by “making sense.”
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Let’s compare two possibilities: first, that our impression of the reality
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around us is basically correct, and second, that reality as we know it doesn’t
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exist and we are being fooled by an evil demon. Our inclination is to collect
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as much information as possible, calculate the likelihood of that informa-
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tion under each scenario, and update our credences accordingly. But in the
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second scenario, the evil demon could be feeding us the same information
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we would expect under the first scenario. There is no way to distinguish
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between the scenarios by collecting new data.
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What we’re left with is our choice of prior credences. We’re allowed to
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pick priors however we want— and every possibility should get some non-
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zero number. But it’s okay to set our prior credence in radically skeptical
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scenarios at very low values, and attach higher prior credence to the straight-
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forwardly realistic possibilities.
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Radical skepticism is less useful to us; it gives us no way to go through
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life. All of our purported knowledge, and all of our goals and aspirations,
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might very well be tricks being played on us. But what then? We cannot
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actually act on such a belief, since any act we might think is reasonable
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would have been suggested to us by that annoying demon. Whereas, if we
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take the world roughly at face value, we have a way of moving forward.
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There are things we want to do, questions we want to answer, and strategies
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for making them happen. We have every right to give high credence to
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views of the world that are productive and fruitful, in preference to those
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that would leave us paralyzed with ennui.
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Some skeptical scenarios aren’t merely fanciful concoctions like Descartes’s
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demon— they are situations that we worry could actually be true. A world
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dominated by Boltzmann Brains is what we would expect if the universe
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were infinitely old and constantly fluctuating. The Matrix was a science-
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fiction conceit, but philosopher Nick Bostrom has argued that it’s more
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likely we are living in a simulation than directly in the “real world.” (The
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idea is essentially that it’s easy for a technologically advanced civilization to
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run powerful computer simulations, including simulated people, so m
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“people” in the universe are most likely part of such simulations.)
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Is it possible that you and your surrounding environment, including all
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of your purported knowledge of the past and the outside world, randomly
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fluctuated into existence out of a chaotic soup of particles? Sure, it’s possi-
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ble. But you should never attach very high credence to the possibility. Such
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a scenario is cognitively unstable, in the words of David Albert. You use your 12
hard- won scientific knowledge to put together a picture of the world, and
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you realize that in that picture, it is overwhelmingly likely that you have
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just randomly fluctuated into existence. But in that case, your hard- won
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scientific knowledge just randomly fluctuated into existence as well; you
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have no reason to actually think that it represents an accurate view of real-
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ity. It is impossible for a scenario like this to be true and at the same time
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for us to have good reasons to believe in it. The best response is to assign it
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a very low credence and move on with our lives.
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The simulation argument is a little different. Is it possible that you, and
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everything you’ve ever experienced, are simply a simulation being conducted
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by a higher level of intelligent being? Sure, it’s possible. It’s not even, strictly
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speaking, a skeptical hypothesis: there is still a real world, presumably struc-
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tured according to laws of nature. It’s just one to which we don’t have direct
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access. If our concern is to understand the rules of the world we do experi-
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ence, the right attitude is: so what? Even if our world has been constructed
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by higher- level beings rather than constituting the entirety of reality, by
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hypothesis it’s all we have access to, and it’s an appropriate subject of study
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and attempted understanding.
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It makes sense, as Wittgenstein would say, to apportion the overwhelm-
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ing majority of our credence to the possibility that the world we see is real,
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and functions pretty much as we see it. Naturally, we are always willing to
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update our beliefs in the face of new evidence. If there comes a clear night,
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when the stars in the sky rearrange themselves to say, “I AM YOUR PRO-
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GRAMMER. HOW DO YOU LIKE YOUR SIMULATION SO FAR?”
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we can shift our credences appropriately.