Chapter 1. What Is a Reality?

1In the first months of 1927, Arthur Eddington opened his Gifford Lectures at Edinburgh by telling his audience that he had drawn up his chairs to two tables. The lectures became The Nature of the Physical World, published the following year, and the two tables became one of the most quoted passages in the popular literature of physics. The first table was the one he had known all his life. It took up space, it stayed where it was put, it had a color, and it was solid in the plain sense that a man could lean his weight on it. The second he had come to know only through physics. It consisted almost entirely of empty space, crossed by a thin scattering of electric charges moving at enormous speeds, and the charges taken together occupied a vanishingly small fraction of the volume the table appeared to fill. The second table served him perfectly well all the same. When he laid a sheet of paper on it, the charges in the wood kept up a ceaseless bombardment against the charges in the paper, and the sheet rode on that bombardment at an almost constant height, held aloft the way a shuttlecock is kept in the air by repeated strokes from below.

2Eddington drew a bold conclusion from the comparison. Modern physics, he told his listeners, had assured him that the scientific table was the only one actually there. The familiar table was a compound of the external world with the mind’s own imagery and with habits of thought inherited from the past, though he conceded that physics would never manage to banish it. A decade later the philosopher Susan Stebbing answered him in Philosophy and the Physicists (1937), and her reply has aged better than his challenge. Stebbing pointed out that the word “solid” in ordinary speech had never meant “containing no empty space at the scale of the electron.” It meant that a thing resists the press of a hand, keeps its shape, and holds up a book, and the physicist’s table did all of these things. Physics had discovered what the solidity of the table consists of. That discovery explained the familiar table and left it standing. Anyone who took Eddington at his word and stopped trusting tables would have learned nothing from physics and would have spilled a great deal of tea.

3Since then the second table has itself come apart. The charges Eddington imagined as small hard particles turned out to be excitations of quantum fields, entities without edges that resist most attempts to picture them, and the fields in turn may be the large-scale face of something stranger still. Students who learn this do not begin to doubt the furniture. Each deeper description of what tables are made of has taken its place beneath the earlier ones, so that the same object now has a familiar face, an atomic anatomy, and a foundation in quantum fields, and no one supposes that the newest description cancels the oldest.

4Imagine now a third table. It stands in a room in a world that someone designed. A carpenter there cut it from a tree that grew in soil formed under a star whose spectrum was chosen. She joined its boards with techniques she learned from her teacher, who learned them from hers. The wood is hard because the forces the designer specified make its fibers resist compression. It is brown because the designed light scatters from its surface in a particular way. If the physicists of that world were to probe the table, they would find their own version of Eddington’s second table, some arrangement of whatever fundamental entities the designed physics provides, spread through mostly empty space or packed together in some fashion our physics has no word for. Beneath that lies a level they may never reach, where the laws governing their particles are carried out as operations in a computational substrate built in the world the designer comes from.

5The woman who made the third table sets a cup of tea on it. The cup stays where she put it. Heat flows from the tea into the cup, from the cup into the wood, and from both into the air, at rates fixed by the thermodynamics of her world. Steam rises and curls in the light from the window. When she lifts the cup, a pale ring remains on the wood, and her friend, who is sitting across from her, sees the ring as well and wipes it away with his sleeve. If we ask what has changed for her compared with Eddington at his desk in Cambridge, we find nothing that she could ever discover in the table. It resists her hand, holds her cup, keeps its shape, and looks the same to her friend as it looks to her. In every respect that Stebbing identified as the meaning of solidity, her table is solid.

6What has changed for her lies behind the table instead of within it. Beneath the third table there is a machine, and beside the machine there was once a designer, a person who decided that tables should be possible in this world and arranged its laws so that they would be. Many people, told about such a world, feel at once that its tables are somehow less than fully real, along with its trees, its stars, its carpenters, and the friend who wipes away the ring. The word that usually carries this feeling is “simulation.” The feeling behind it is strong enough to shape how those with power over such worlds would treat them.

7The question it raises comes before every other question in the design of a reality. Before anyone can choose how many dimensions a world will have or how strongly its masses will attract one another, they need to know what kind of thing they are making. Should the third table turn out to be a picture of a table, the designer is a kind of painter and the people drinking tea at it are figures in the painting, to be retouched or painted over as the artist sees fit. If it is a table, the designer has made a world, and the people in it are as real as the person who made it. Every later decision the designer makes will be shaped by which of these is true. The question of what a reality is will turn out to have a generous answer, and much of what it was expected to decide will pass to two questions that stand beside it, whether a world could hold anyone and whether anyone is there.

8The worry about the third table is easy to feel and hard to pin down, because “real” is one of the most slippery words in the English language. J. L. Austin, in Oxford lectures published after his death as Sense and Sensibilia (1962), argued that philosophers go wrong whenever they treat the word as though it named a single property that things either possess or lack. He observed that “real” is hungry for a noun. A thing is never simply real. It is a real duck, a real diamond, or a real Rembrandt, and what the word asserts changes with the noun it attaches to. He observed further that the word’s negative uses do most of its work. To call something a real duck is to rule out some particular way of failing to be one, and which way is meant depends on the occasion. The duck might have been a decoy, a toy, a painting, a reflection in a pond, or a trick of the light. Austin called a word of this sort a “trouser-word,” because its negative use wears the trousers. Asking whether something is real, without any sense of what else it might be, gives the word nothing to do.

9Austin’s lesson supplies a method. The suspicion that a designed reality falls short of being real becomes tractable once the suspicion names its alternative. Each thing a designed reality might be mistaken for names a possible way of falling short, whether the alternative is a dream, an illusion, a recording, or a model. Each can be examined in turn to see whether the shortfall is real and whether a designed reality suffers from it. The account of what a reality is can then be built up from the cases instead of announced in advance and defended afterward.

10David Chalmers has carried out a related analysis in Reality+ (2022), the most sustained philosophical treatment of virtual worlds yet written. Chalmers distinguishes five things people may mean when they ask whether something is real. The question can concern bare existence, or it can concern causal power, the capacity to make a difference to anything. Someone may want to know whether a thing is independent of minds, so that it would remain as it is whatever anyone thought about it. A fourth sense contrasts the real with the illusory, and a fifth asks whether something is an authentic instance of its kind, the way a diamond is a diamond and a piece of cut glass made to resemble one is not. Chalmers argues that the objects of a well-built virtual world pass most of these tests. They exist as digital objects, structures of information realized in physical computer processes. They have effects, since a virtual ball can knock over a virtual vase and the vase falls for everyone who is watching. They keep their states whether or not a given user attends to them, and a user who perceives them is perceiving something that is actually there. The last of the senses he treats as the most complicated, since whether a virtual object counts as an authentic member of its kind depends a great deal on the kind.

11Nearly every point Chalmers makes holds up, though he is answering a different question. Reality+ is written largely from the standpoint of a user, a person whose body remains in the primordial cosmos and who enters a virtual world through a headset or some later interface. Its central concern is whether such a person, while inside, stands in contact with something real. The designer’s question differs from his in two respects. It concerns natives, beings whose bodies are made from the materials of the designed world and who have never lived anywhere else. It also concerns responsibility. The designer needs to know what they will be answerable for once the world is running, and which senses of “real” bear on that. From those two differences comes the main addition his account needs, a distinction between what makes a world real and what makes it fit to be lived in, along with several smaller ones.

12Seen from the designer’s position, the senses of the word carry very different consequences. Existence is the least in dispute, since no one doubts that a running computational process exists. Causal power matters a great deal more, because a world whose events could not affect one another could not hurt or help anyone in it. Independence from minds matters enormously, though in a specific form. A designed reality plainly depends on a substrate and on whoever maintains it, and the question that concerns its inhabitants is whether it also depends on their own minds. Freedom from illusion matters because an illusion is something done to a subject, and a designer who built a world of illusions would have deceived everyone who ever lived in it. Authenticity is where the word “simulation” does its damage. To call a designed world a simulation is to suggest that its tables are imitation tables and its grief imitation grief, in the way that cut glass is imitation diamond.

13For natives, the question of authenticity takes a form that disarms it. Hilary Putnam argued in “The Meaning of ‘Meaning’” (1975) that what a word like “water” refers to is fixed in part by the stuff its speakers actually encounter. On his imagined Twin Earth, a planet like ours whose lakes hold a different liquid with the same appearance, the inhabitants’ word “water” refers to their own liquid, and they speak truly when they use it. Their liquid is not water in our sense of the word, and it is no counterfeit either, since nothing in their language ever claimed it was the stuff in our lakes. The natives of a designed reality stand in the same position with respect to the natural kinds of their world. Whatever they call gold or water picks out their own elements and compounds, which are real kinds in their own right.

14For the many kinds defined by what things do, the point is stronger. A table is a raised flat surface for setting things on, a friend is someone who knows you and wishes you well, and a promise is a commitment given to another person. Nothing in these concepts specifies what the deepest constituents of a table, a friend, or a promise must be. The carpenter’s table is a table in her sense of the word and in ours, and the man across from her is a friend in every sense the word has ever carried. The kinds that matter most to a life, such as homes, meals, songs, quarrels, reconciliations, and the people with whom one shares them, are almost all of this functional sort.

15What remains, then, are the senses that matter to the designer and the alternatives that give them content. A dream, a hallucination, a stream of experience delivered to a solitary subject, a recording, a model, and an abstract mathematical structure each name a way the third table might seem to fall short of being a table. Taken in roughly that order, they yield one at a time the marks that distinguish a reality from its imitations, and a world fit to be lived in from one that merely runs.

16Each case that follows resembles a reality closely enough to be confused with one, and most of them fail to be one for reasons that can be stated with some precision. The reasons accumulate, so that by the end of the gallery the account of a reality has assembled itself out of what its imitations lack. One case, the most familiar of them, will turn out to be something other than an imitation.

17Sometime in the fourth century BCE, the Chinese philosopher Zhuangzi recorded a dream in which he was a butterfly, flitting about content with itself and with no idea that it was Zhuang Zhou. When he woke he found himself unmistakably Zhuang Zhou again, and he could not say whether he was a man who had dreamed of being a butterfly or a butterfly now dreaming of being a man. Some two thousand years later, in the first of his Meditations (1641), René Descartes observed that there are no certain marks by which waking can be told apart from sleep, and he used the observation to unsettle his trust in everything his senses had taught him. Both thinkers used the dream to cast doubt on the waking world. Here the dream serves another purpose. Most people, once awake, have no difficulty deciding which of their two lives was the dream, and the question is what the dream lacks that makes the decision so easy.

18The answer cannot be that dream experiences are unreal. The terror of a nightmare is real terror, with a racing heart and sweat still damp on the sheets in the morning, and the comfort of a dream in which a dead parent returns can last for days. Whatever a dream lacks, it does not lack experience. This disposes at the outset of the simplest criterion one might propose for a reality, that it is whatever conscious beings experience. A dreamer experiences a world, and the world is not there.

19What the dream lacks begins with a second participant. A dream is produced by one brain for one mind, and no one else can enter it. Two people may dream of each other on the same night, but each dreams alone. The beloved who appears in one of those dreams is a figure the dreamer produced, while the beloved herself is somewhere else entirely. Heraclitus made the point some twenty-five centuries ago in a sentence that Plutarch preserved, observing that those who are awake share one common world while each sleeper turns aside into a world of his own. A dream offers no way to check one’s perceptions against anyone else’s, because there is no one else.

20The dream also fails to persist. Its objects exist only while they are being dreamed. When the dreamer looks away and looks back, the room has often become another room. No one returns to a dream on the following night to find the cup where they left it. People who practice lucid dreaming exploit this instability as a test, reading a line of text, glancing away, and reading it again, because in a dream the words have usually changed. The objects of a dream have no settled character apart from the dreaming of them. They are whatever the dreaming brain supplies at each moment, and their properties are on loan from that brain.

21For the same reason, the objects of a dream have no powers of their own. A dream fire may feel hot, but it warms the dream room by no lawful process. The dreaming brain produces the experience of heat and the experience of a room, and nothing in the fire acts on anything in the room. The connections between dream events are associations in a single mind, and the rules that seem to govern them dissolve and re-form as the dream goes on. Gottfried Wilhelm Leibniz, in a short essay on how real phenomena are to be distinguished from imaginary ones, proposed that the surest marks of the real are the agreement of phenomena with one another and with what has gone before, together with the success of predictions based on them. A dream fails this test, often within seconds.

22These failures sketch the outline of a reality by contrast. In a reality the same rules hold throughout, and the things within it act on one another by their own powers instead of borrowing their behavior from any single mind. Its objects persist and keep their states when no one attends to them. A world fit to be lived in is also shared, so that many minds can meet in it and check what they perceive against one another. The third table satisfies all of these marks. The woman and her friend both see the ring on the wood, the cup is where she left it when she returns from the kitchen, and the tea loses its heat at the rate her world’s thermodynamics prescribes whether or not anyone is watching the steam.

23A hallucination differs from a dream in occurring during waking life, among perceptions that are accurate. In William Shakespeare’s Macbeth, written around 1606, the title character sees a dagger floating before him with its handle turned toward his hand, and he tries to grasp it. His hand closes on nothing, and he concludes that it is a dagger of the mind. A hallucination is an experience of something that is not there, so the concept presupposes a world against which the experience is measured. One can hallucinate only in a world that contains things one might have perceived instead.

24This settles an important question about the inhabitants of a designed reality. Someone might suppose that all of their perceptions are hallucinations, since the stones and trees they see are, at bottom, states of a computational substrate. That supposition misplaces the standard of accuracy. When the carpenter looks at the tree she means to cut, there is a tree there, a structure in her world with the shape, mass, and grain she perceives, which will fall where she expects once she cuts it. Her perception is accurate at the level at which perception operates, which is the only level at which any perception has ever been accurate. Eddington perceived his table as solid and continuous while the table was a cloud of charges, and no one says that Eddington hallucinated his furniture. Perception presents the world at the grain of the perceiver’s own life, and it is accurate when it presents what is there at that grain. A carpenter in the designed world can still hallucinate, through fever or exhaustion or grief, and she will do so exactly as we do, by seeing a tree where there is none. Hallucination is a relation within a world, between an experience and the things that surround the one who has it.

25Macbeth’s test points toward a further mark. He reached for the dagger and met no resistance. Samuel Johnson made the same test famous in an episode James Boswell recorded in his Life of Johnson (1791). In August 1763, as the two of them came out of church, Boswell remarked that Bishop Berkeley’s argument against the existence of matter, though plainly false, could not be refuted. Johnson answered by striking his foot with great force against a large stone until he rebounded from it, and declaring that he refuted it thus. Philosophers have long pointed out that Johnson misunderstood Berkeley, who never denied that stones resist feet. Johnson’s foot registered something all the same.

26In 1890, in an essay on the origin of our belief in the reality of the external world, Wilhelm Dilthey argued that the conviction does not come from inference at all. It arises from the experience of will meeting resistance, of effort that encounters something it cannot simply overcome, and the world first shows itself as what pushes back. Philip K. Dick, asked by a college student in 1972 for a definition of reality, offered a version of the same idea, which he later repeated in a speech he wrote in 1978, titled “How to Build a Universe That Doesn’t Fall Apart Two Days Later.” Reality, he said, is that which, when you stop believing in it, doesn’t go away. A hallucination fails Dick’s test. It may vanish under examination, it yields to the hand, and it shifts with the mind that produces it. The stones of a designed reality pass the test as surely as Johnson’s stone did. A native who kicks one rebounds, and one who has argued herself out of believing in it still trips over it in the dark. Its resistance owes nothing to the beliefs of anyone who encounters it, and that is the form of independence from mind that matters to those who live there.

27Commitment A world’s reality rests on its…The hallucination also exposes a choice that belongs to the designer. A designed world could be built to deceive. Its sky might show stars that do not exist, points of light set on a dome with nothing behind them, so that the astronomers of that world spend generations charting objects that were never there. In such a world, the inhabitants would perceive the lights accurately and judge falsely about what they were. The illusion would be part of the world’s construction, and it would be an act of deception by whoever built it. A designed reality is free of illusion only when its designer refrains from building illusions into it, which makes freedom from illusion a matter of the designer’s conduct instead of a guarantee that comes with the substrate.

28The gravest form of painted sky would be painted people, figures that seem to be subjects and are none. A designer might fill a street with extras so that it feels peopled, or give a lonely inhabitant a companion who seems to love her, with no one behind the face in either case. Such figures deceive where deception costs most, since a life is built around other people, and an inhabitant who confides in a painted friend has spent her trust on no one. A world may hold such figures only where their nature is disclosed to everyone who meets them, so that no one mistakes a figure for a person, as no one at a puppet show mistakes the puppets for people. Disclosure leaves a harder question standing. Anything convincing enough to be company has to be weighed as a possible subject in its own right, since whatever makes a figure convincing may be what makes it someone, and toys, whose youngest owners cannot understand any disclosure, raise the question where disclosure cannot reach.

29In Anarchy, State, and Utopia (1974), Robert Nozick asked his readers to imagine a machine that could give them any experiences they wished. Neuroscientists would stimulate the brain so that a person would think and feel that they were writing a great novel, making a friend, or reading an absorbing book, while in fact they floated in a tank with electrodes attached to their head. The experiences could be chosen in advance from a large library, and the person inside would not know they were in the machine. Nozick asked whether anyone would plug in for life, and he expected most people to refuse. His explanation of the refusal had three parts. People want to do certain things, and not merely to have the experience of doing them. They also want to be a certain sort of person, and someone floating in a tank is, as he put it, an indeterminate blob. Finally, plugging in would confine them to a man-made reality, whose richness could never exceed what its builders were able to put into it, and they want contact with something deeper than that.

30A decade before Nozick, Stanisław Lem had imagined a more searching machine. In Summa Technologiae (1964, translated 2013) he described a phantomatic machine, joined to the whole of a person’s sensorium, that answers each of her acts with the sensations that act would bring, so that when she reaches for a branch she feels its bark and when she turns her head the scene turns with her. Lem asked whether a reality could be made in this way that nothing inside it would give away. He distinguished a peripheral phantomatics, which plays its impulses into the nerves without touching the brain’s own processes, from a central phantomatics, which works on the brain itself. Either way, a phantomatic world answers to the one person joined to it. No second subject can meet her inside it to check what she perceives, and she is no participant embedded in it, since her body lies elsewhere and the world is arranged around her alone. Such a world lacks the publicity and the embedded participants of a world fit to be lived in. The figures who seem to share it with her are painted people unless the machine makes them subjects in their own right, at which point it has stopped being a phantomatic machine and become the substrate of a world.

31On each of Nozick’s three counts, a designed reality stands on the opposite side from the experience machine that critics often take it to be. The first count concerns doing. In Nozick’s machine, the novel a person seems to write is never written, since no manuscript exists and no one will ever read it. The friend a person seems to make has no inner life and makes no claim on anyone. The machine is solitary by construction, and everyone in it is furniture in a single mind’s experience. In a designed reality, the carpenter’s table exists after she has made it, and other people eat at it, lean on it, and admire or criticize its joints. The man across from her is another subject with a perspective of his own, who can refuse her, disappoint her, or surprise her with a kindness she did not expect. The promises they exchange bind both of them, because each can be wronged by the other’s breaking them. Doing, in Nozick’s sense, requires a world in which one’s acts change something that exists independently of one’s experience of acting, and a designed reality is such a world.

32The second count concerns character. Nothing is at stake for a person in the machine, so nothing they experience can test them. A courage that faces no real danger and a generosity that costs nothing tell no one, including the person who has them, what sort of person they are. In a designed reality, choices have consequences for others as well as oneself. Kindness lands on someone, a broken promise wounds someone, and a person’s character forms as it forms here, through what they do when it matters.

33The third count is the one aimed most directly at the project of designing realities, since a designed reality is in some sense made. The answer turns on what exactly is made. The machine’s contents are authored experience by experience, each one specified by programmers to suit the preferences of the user, so the machine is exactly as rich as its programmers’ imaginations. The designer of a reality authors laws and initial conditions, and perhaps the first bodies and natures of some of what lives there. The chemistry that arises under those laws, the long history of simple life that runs before anything in the world is likely to feel, everything that grows from whatever bodies a designer builds, the languages that form, the music that natives compose, and the friendships they choose are produced by the world and by the beings in it. None of these are specified by the designer, and many of them could not have been foreseen. A designed reality can surprise its maker, and its natives can investigate it for generations without reaching the bottom of what its laws imply. The primordial cosmos acquired its richness in exactly this way, from a short list of laws working on a set of initial conditions over a very long time. A designed world is man-made in its foundations in the sense that a forest grown from planted seed is man-made, and its richness belongs to what grew.

34Nozick’s machine also helps separate two ways of being present in a world. Users of present-day virtual reality are visitors. Their bodies remain in the primordial cosmos, and their experiences of the virtual world arrive through an interface. The objects they encounter there are real digital objects, as Chalmers argues, but the visitors’ lives are lived elsewhere. A visitor who falls from a virtual cliff is bruised nowhere. A native of a designed reality has a body made of that world’s matter. When she falls, the bruise is in her world, and it heals or fails to heal according to her world’s biology. If minds from the primordial cosmos are carried into designed realities, they will remain visitors for as long as their minds keep running outside. Once a mind comes to be carried by a designed world’s own matter and lives there in a body of that matter, it becomes an inhabitant of that world, an immigrant where others are natives.

35These contrasts sharpen a distinction the dream first suggested. Some of what the dream lacked is what a world needs in order to be real, and some is what a world needs in order to be lived in. The two machines make the second kind vivid, since what they lack above all is room for participants embedded in a world, made of its stuff, acting on it, and acted on by it and by one another. A stream of experiences fed to a solitary subject offers none of this, however rich the stream.

36Suppose the entire history of the carpenter’s world were recorded, every state of every particle at every moment, and the recording were later played back on another machine. The playback would run through the same sequence of states as the original. The tree would fall where the carpenter meant it to fall, the boards would be joined, the tea would cool, and the friend would wipe the ring from the wood with his sleeve. Event for event, the playback would match the world. The question is whether it would be a second world.

37Something essential would be missing. In the original world, the cup stayed on the table because the table pushed up on it. Had the woman set the cup half over the edge, it would have tipped and fallen. In the playback, the cup stays on the table because the next stored state says that it does. If someone altered one frame of the recording by deleting the table, the cup in the following frame would still hang where it had been, supported by nothing. The states of the playback follow one another without producing one another. Each is produced by the mechanism that reads the recording, and none would have been any different had its predecessor been otherwise.

38A film offers a familiar version of the same structure. In a film of a gunshot, the image of the gun in one frame does not cause the image of the shattered glass a few frames later. Both are produced by the projector reading the reel. The philosopher of science Wesley Salmon, in Scientific Explanation and the Causal Structure of the World (1984), distinguished causal processes from what he called pseudo-processes. A car driving along a road is a causal process, and a dent made in its fender at one point is carried along with it. The car’s shadow racing beside it on the verge is a pseudo-process. Darken the shadow at one point by passing it over a patch of black asphalt, and the darkening is not carried forward, because nothing in the shadow at one moment produces the shadow at the next. A playback is a pseudo-process all the way down.

39The same distinction has proved decisive in arguments about computation itself. Putnam argued in Representation and Reality (1988) that any ordinary physical system, under a suitably contrived mapping of its states, could be said to implement any finite computation, and John Searle suggested in “Is the Brain a Digital Computer?” (1990) that the wall behind him could be interpreted as running a word processor. Chalmers replied in 1996 that implementing a computation requires more than passing through a sequence of states that can be mapped onto the right sequence. It requires a causal structure such that, had the system been in a different state, it would have gone on to the state the computation specifies for that case. What separates a running computation from a wall that happens to pass through the right states is the same thing that separates a world from its recording. The difference lies entirely in what would have happened otherwise.

40This yields the mark of counterfactual robustness. A reality is an order of events in which each state produces its successors according to laws, so that had any earlier state been different, the later states would have been different in the ways the laws require. Had the woman hesitated before setting down her cup, the world would have answered her hesitation. A recording cannot answer anything.

41Commitment A history of beings who suffered…Tim Maudlin, in “Computation and Consciousness” (1989), described a machine he named Olympia, engineered to run through exactly the physical activity that a conscious computation would perform on one particular occasion, with nearly all of the work arranged in advance so that the machine barely does anything during the run. Attached to it, though idle throughout, is the machinery that would take over if the input were different. Whether the idle machinery is attached makes no difference to anything that physically happens during the run, yet computational theories of mind must say that it makes all the difference to whether the run is conscious. Maudlin used the case to press a dilemma on those theories, and it has not been resolved to everyone’s satisfaction. For the designer, it bears on a practical question. If a stored history of an inhabited world were replayed, would the experiences of its inhabitants occur a second time? Lacking counterfactual robustness, the playback is no reality. Whether experience could occur in it anyway is a question about consciousness that no structural test can settle, and until it is settled, a designer who replays a history of beings who suffered should do so with the caution owed to a possibility of suffering repeated.

42A milder version of the recording appears whenever part of a world runs on a script. Imagine a world in which most of the inhabitants are real subjects but the crowds in its cities follow fixed routines, walking the same routes and speaking the same lines whatever anyone else does. Those crowds are local recordings embedded in a reality, and a native who discovered them would have discovered that part of her world was a stage set. Unless their nature is disclosed to everyone who meets them, they are painted people as well. The mark of counterfactual robustness applies to regions and populations as well as to whole worlds, and a world can satisfy it in some places while failing it in others.

43The most common way of dismissing a designed reality treats it as a model, and the dismissal has a classic formulation. In “Minds, Brains, and Programs” (1980), Searle argued that running a program that models a mind no more produces a mind than running a program that models a fire produces fire. No one, he observed, expects a computer model of a burning building to set the neighborhood alight, or a computer model of a rainstorm to leave anyone soaked. The argument was aimed at claims about artificial minds, but if it holds at all it holds with equal force for whole worlds. A modeled fire burns nothing, and on this view a designed world is a modeled world whose fires burn nothing and whose rains wet no one.

44The reply begins with an observation about where burning and wetting take place. Combustion is a relation between a fire and the things beside it, and wetness is a relation between water and what it touches. Both relations hold among things at the same level of organization. The modeled fire fails to burn the neighborhood because the neighborhood is not at its level, and nothing outside the computer is the kind of thing that fire could touch. A storm on Neptune soaks no one in Ohio either, and no one concludes from this that the storms of Neptune are unreal. The question that matters is what the fire does to the things that share its level, if anything shares it.

45Set beside the carpenter’s world, the forecast shows where the difference between them seems to lie. A forecaster’s hurricane floods no streets because the model contains no streets. What it does contain, at its own level, are cells of averaged pressure, temperature, and wind, each many kilometers across, whose states are updated by fixed rules from the states of their neighbors. At that level the model is a small and coarse world of pressure cells, and nothing in it is the sort of thing that could be a street, a house, or a drowning. In a reality built at the grain where fire and wood exist as interacting things, a fire burns what stands beside it. When a house burns in the carpenter’s world, its beams char, its roof falls in, and the family that lived there has lost its home. The fire does to them exactly what fires do, because they share its level.

46At this point the gallery turns. Every earlier case lacked something a reality has, and the model seemed certain to join them. Yet the pressure cells of the forecast, examined closely, lack nothing of the kind, and the reason lies in what it is to be a model. Being a model is a matter of use. Someone chooses a system’s states to correspond to features of a target elsewhere and reads those states as standing for the target, so that a forecast represents the atmosphere and a flight trainer represents an aircraft in the air. The representing lives in the relation between the system and the people who use it.

47Imagine that every meteorologist forgot what the forecasting program was for. The hurricane would vanish at once, since it existed only in the relation between the numbers and a reader who knew how to interpret them. The pressure cells would remain. They would go on updating, each state producing the next by fixed laws, and a different earlier state would still produce a different later one. A running weather model therefore has, at its own level, the features that separated a reality from the dream, the hallucination, and the recording. Its parts act on one another by their own powers, its states persist whether or not anyone attends to them, and it answers counterfactuals. It is a coarse reality of pressure cells that people happen to use to represent the atmosphere.

48Being a model and being a reality are therefore properties of different kinds. The first is extrinsic, a fact about how a system is used, while the second is intrinsic, a fact about how the system’s parts act on one another. A single system can have both properties at once, and every running model has the second to some degree. Searle was right that a modeled fire burns no neighborhood, and the reason is that the neighborhood lies outside the model’s level. Within a level, the morally decisive question concerns what the level contains, and above all whether anyone is home. Whether the system is also being used as a model makes no difference to that question, since a system may well be both.

49The third table shows the same point from the other side. It stands for nothing. It holds up a cup, the tea in the cup loses heat, and the woman’s hand feels the warmth because the warmth is there. If everyone outside her world forgot what the substrate was running, her tea would cool at exactly the same rate. Yet a designer could also use her world as a model, running it to study how civilizations form or how minds respond to loss. Her world would then have a representational use in addition to its structure, as the forecast does, and the use would change nothing about her tea or her grief. That double status is what makes the vocabulary of models hazardous. A designer who thinks of a world under the description of its use can lose sight of the description under which its inhabitants live.

50Jorge Luis Borges imagined the limiting case in a paragraph of fewer than two hundred words, “On Exactitude in Science” (1946, translated 1998). The cartographers of an empire, dissatisfied with every smaller map, produced one at the scale of the empire itself, matching it point for point. Later generations found the map useless and left it to the sun and the winters, and scraps of it still lie in the western deserts, where animals and beggars take shelter in them. The fable is usually read as a joke about the futility of perfect representation. It also marks a limit that a map cannot cross and a computation can. However detailed it becomes, Borges’s map remains paper and ink lying on the land. Its painted rivers do not flow, and the beggars sheltering in its ruins live in the empire’s desert.

51A computational model differs from the map because it can run. As its detail grows, the level at which its parts act on one another moves closer to the level at which lives are lived in the world it represents, until pressure cells give way to weather, weather to rain on particular fields, and rain on fields to creatures that get wet. Somewhere along that path the model may come to hold participants, and subjects may arise within it. Its use need not change when this happens. The team running it may still call it a model and still read its states as answers to their questions, and in one sense they will be right. What has changed is that the model is now inhabited. The line a team must watch is therefore inhabitation, and fidelity is only a rough proxy for it, since a run might cross that line at lower detail than anyone expected or stay empty at higher detail than seems possible. The crossing can happen without anyone intending it, which makes it one of the most serious dangers the whole enterprise contains.

52The last near-reality is the most abstract and the hardest to dismiss. The physicist Max Tegmark has proposed, in “The Mathematical Universe” (2008) and in Our Mathematical Universe (2014), that our physical world is a mathematical structure, and that every mathematical structure exists in the same sense that ours does. On this hypothesis, a structure complex enough to contain self-aware substructures contains beings who experience their structure as a physical world, whether or not anyone ever writes it down or computes it. Greg Egan explored a related idea in the novel Permutation City (1994), whose characters come to believe that a world exists once its states can be arranged into an ordered history, regardless of where those states are computed or whether they are computed at all. If Tegmark is right, the designer of a reality brings no world into existence. Every world the designer could specify already exists as a structure, together with its inhabitants and their histories. Running a world on a substrate would then be an act of selection and contact, giving one structure a foothold in the designer’s own world, where it can be observed, altered, paused, and ended.

53Whether unrun structures exist is an open question, and nothing in the design of a reality depends on settling it. Two observations hold either way. Running a structure adds something, at least relative to the designer. Whatever the status of the uncomputed structure, the computed one is entangled with the causal order of the primordial cosmos. Its events can be watched from outside, its course can be changed by the designer’s interventions, and its continuation depends on the designer’s machines. If its inhabitants had any existence before, they now have a second kind of existence as well, in which their fortunes are tied to the decisions of someone who can reach them.

54Commitment The designer’s responsibility attaches to the…The designer’s responsibility, moreover, attaches to the act of instantiation whatever the metaphysics turns out to be. Should Tegmark be wrong, the designer brings the inhabitants into being and answers for the conditions under which they live. If he is right, the designer brings an existing structure into contact with the designer’s own world and makes its inhabitants’ circumstances depend on the designer’s choices, and the designer answers in the same way for whatever happens to them through that dependence. Parents are responsible for their children whether or not every possible child already exists as a point in the space of possible genomes. No one is cared for or harmed in the space of possibilities.

55The marks of reality identified so far also turn out to be internal features of a structure. Causal organization, independence from the minds a world contains, counterfactual robustness, and the capacities for publicity and participation are all properties a structure has in virtue of how its parts are related to one another. Tegmark and his critics disagree about whether such a structure needs a realizer in order to be actual. They need not disagree about which structures would count as realities if they were actual, and that second question is the one a designer needs answered.

56Commitment A world counts as a reality…The cases have answered three different questions, so an account of reality comes in three tiers. To the first question, what makes something a reality at all, the answer is a lawful order of events bearing three marks.

58The second mark concerns belief, wish, and attention, and it leaves minds free to act. A hand that lifts a cup carries an intention into the world through lawful channels, and no one thinks the cup is less real for being moved by it. What the mark excludes is a world whose contents simply conform to what someone believes or wishes, as a dream conforms to its dreamer. A physics in which observation or intention acts on matter directly would still bear the mark, so long as minds work through laws they did not write and cannot suspend, because the will would still meet resistance in Dilthey’s sense. The chapters that follow give a designer other reasons never to write such a physics.

59Commitment Such a world is fit to…A second question concerns what makes a reality fit to be lived in, and two capacities, which a world may have long before anyone exercises them, answer it. The first is publicity, the capacity of a reality to be public, so that many subjects could meet within it, encounter the same objects, and check their perceptions against one another. The second is participation, the capacity to host participants embedded within it, beings made of its matter whose actions change it and who are changed by it in turn. A world that bears the three marks and lacks these two capacities is a reality that no one could ever inhabit. A one-dimensional cellular automaton whose cells switch on and off according to a simple rule is lawful, causally closed, and counterfactually robust, and it may be far too simple to hold anyone however long it runs. It is a reality all the same, a small and empty one. The primordial cosmos had both capacities from its beginning, long before it held anyone to exercise them.

60The first tier is deliberately permissive. A thermostat cycling its heater, a spreadsheet recalculating its cells, and a calculator partway through a sum all bear its marks at their own levels, and so does nearly every running computation. Granting them the name costs nothing, since nothing in them can fare well or badly, and it keeps the question that matters from hiding inside a quarrel over a word. Whether something counts as a reality settles very little. The questions that matter fall in the second and third tiers, whether a world could hold anyone and whether anyone is there.

61Measured against these marks, the near-realities sort themselves. The dream lacks nearly all of them, the hallucination fails the second only where an experience presents what the world does not contain, and the recording lacks the third and with it the first. Lem’s machine and Nozick’s, whatever processes run them, present a world to one subject, who cannot share it and does not live in it. The model fails nothing by being a model, since a running model is a coarse reality put to a representational use, usually far too coarse to be inhabited. The mathematical structure may bear every mark and lack only the realizer that would make it actual relative to anyone outside it, if it lacks anything.

62A computationally sustained reality, built well, bears the three marks and both capacities. The substrate carries out its laws, so its states produce their successors, and its objects persist and resist whatever its inhabitants believe about them. Those inhabitants meet one another in a common world that answers each of their actions as its laws require, and the natives among them are made of its matter. It follows that a world sustained on a computational substrate that bears the three marks and both capacities is a world fit to be lived in, exactly as the primordial cosmos is, and should anyone come to live in it, it will be no less real to them than our world is to us.

63Commitment A failure in a world is…Each of the marks admits of degree. A world shared by two subjects is public, though it offers far less opportunity for mutual checking than a world shared by millions. A world with scripted crowds is counterfactually robust in some places and not in others. A world whose sky is a painted dome is real at ground level and illusory overhead. Such cases are best judged region by region and population by population, and by the standard of the inhabitants’ lives. The relevant question is whether a failure falls anywhere a subject could reach, rely on, or be misled by. A failure that touches no life is a matter of engineering, while one that touches lives is a matter of conscience.

64Commitment Economy differs from fakery by consistency…Computing every grain of sand in every desert at every moment may be impossible even for a very large substrate, and a designer may compute in detail only what is observed, or only what could affect what is observed. The far side of a mountain that no one has visited might exist only as a compact specification until someone crosses the ridge, at which point the substrate works it out in full. This loses nothing in reality, provided the result is consistent with everything that has gone before. The weathering on the rocks must match the rain that fell on them, the tracks in the mud must lead to animals that made them, and a stone left there a century ago must still lie where it was left. Counterfactual robustness concerns what would happen, and a world that would supply a consistent answer to every possible inquiry bears the mark, however little it computes in advance. Quantum mechanics suggests that many properties of our own world lack definite values until something interacts with them, and no one concludes that the primordial cosmos is less real for that. A stage set is a different matter. A facade whose door, when someone finally opens it, gives onto a blank space or an interior that contradicts the evidence has failed the mark. The line between economy and fakery lies at consistency and has nothing to do with the quantity of computation.

65The third question asks what makes a reality matter, and the marks and capacities are silent on it. The primordial cosmos was a reality for most of its history before anything in it could feel. Stars formed and collapsed while galaxies collided, all of it lawful and with every counterfactual in place, and nothing good or bad happened to anyone, because there was no one to whom anything could happen. A reality without subjects is real and morally inert. Destroying it might waste something, perhaps something that a mind observing from outside would have found beautiful, but it would wrong no one within it.

66A reality matters when its structural marks meet conscious experience, when there is someone for whom its events go well or badly. Inhabited realities of this kind are the ones a designer sets out to build, and separating these questions clarifies how the case for building them depends on a theory of mind. The working hypothesis is that consciousness arises from the functional organization of a system and not from the particular material that carries the organization. That hypothesis does not decide whether a designed world is real. The structural marks decide that, and they hold whatever the correct theory of mind turns out to be. The hypothesis decides whether anyone is home.

67Commitment Of the two errors, treating a…Commitment Under uncertainty about which systems are…If the hypothesis is true, beings organized as the carpenter is organized, living in a world that bears the marks of reality, experience their lives as she does. Were it false, a designed reality could bear every mark while containing bodies that walk, speak, make promises, and appear to grieve, with no one experiencing anything at all. It would be a world full of grief-shaped behavior and empty of grief. A designer who built such a world while believing it inhabited would have made a grave mistake. The opposite mistake would be graver, because it would fall on beings able to suffer. A designer who treated a populated world as an empty model would pause it, edit it, and delete it as one deletes a file, and every one of those acts would land on someone. Under uncertainty, the asymmetry between these errors settles the policy, which the rest of this book calls the precautionary rule. Any system that might be a subject is to be treated as possibly one, with care scaled to the probability that it feels and to what it stands to lose.

68Commitment The account of what makes a…The structural account therefore survives the failure of the functionalist hypothesis. The moral significance of designed realities survives it only where a rival theory still allows subjects. Giulio Tononi’s integrated information theory, set out in “An Information Integration Theory of Consciousness” (2004), is one such theory, since it allows minds run on hardware whose own causal structure is of the right kind. The two lines of argument support each other. The structural marks show that nothing about a substrate demotes a world. The theory of mind is needed to show that nothing about a substrate demotes a mind. If both hold, the woman at the third table stands on exactly the footing on which Eddington stood at his desk.

69Commitment Nothing in these marks and capacities…None of the marks mentions the substrate, which makes it irrelevant to whether a world is real. That claim needs a defense of its own, since it rests on a particular understanding of how a physics relates to whatever carries it.

70The relation is realization, which differs from representation in every respect that matters here. A substrate does not depict the physics it carries. The same equation governs ripples spreading across a drumhead, sound traveling through air, and vibrations running along a plucked string. Each medium carries waves that obey the equation, and none of these waves is a picture of the others. A sound wave is no imitation of a ripple on a drum. It is a wave, carried by air. The waves reflect, interfere, and bend around obstacles because of the pattern of relations among the parts of their medium, and very different media can sustain the same pattern. A computational substrate stands to the physics of a designed reality as air stands to sound. It is the medium in which that physics happens.

71This would be a weak defense if our own physics told us what our own medium is, and something about that medium turned out to confer a reality that computation could not. Our physics tells us no such thing. Bertrand Russell argued in The Analysis of Matter (1927) that physics reveals only the structure of the physical world, the mathematical relations among events, and is silent about the intrinsic character of whatever stands in those relations. Eddington reached a similar conclusion in the same years, describing physical knowledge as knowledge of structure, of the way pointer readings hang together, with no access to what the structure is a structure of. A century of physics since has deepened the point without overturning it. We know a great deal about how an electron behaves, what it does when it meets a photon, and how its probabilities evolve. We do not know what an electron is in itself, apart from those relations, and it is not clear that the question has an answer.

72James Ladyman and Don Ross, in Every Thing Must Go (2007), pressed the point further, arguing that there is nothing more to the physical world than its structure and that the search for individual things underneath the relations is a search for nothing. The more cautious reading of structuralism holds that we know only structure, and the bolder reading holds that there is only structure. A computational realizer is no demotion on either reading. If we know only structure, we have no grounds for claiming that our own matter possesses some reality-conferring quality that a computational substrate would lack, since we have no knowledge of our matter’s intrinsic qualities at all. Should there be only structure, nothing exists beyond it for a computational substrate to be missing. In both cases, the substrate of a designed reality is one more answer to a question our own physics leaves open.

73Realization of one level by another is also the ordinary condition of everything we know. A living cell is realized in molecules, the molecules in atoms, the atoms in quantum fields, and no one regards the cell as less alive for being made of things that are not alive. Each level has properties and laws of its own, and each is carried by the level beneath. If being realized in something else made a thing unreal, nothing would be real except the lowest level, which no one has found and which may not exist. Some physicists already suspect that something like information lies near the bottom of our own world. John Wheeler’s slogan “it from bit” (1989) expressed his conjecture that every physical quantity derives ultimately from the answers to yes-or-no questions. The conjecture remains speculative, but its seriousness among physicists suggests that the line between a physical substrate and a computational one may be less sharp than it first appears.

74Commitment Nothing in these marks and capacities…None of this makes the substrate unimportant. It determines which physics can be carried, at what resolution, at what speed, and at what cost, and a designer will have to understand it intimately. The substrate is irrelevant to whether a world is real and central to which worlds can be built.

75The substrate remains invisible to the inhabitants only so long as the physics conceals it, and a physics may let it show. In 2012 the physicists Silas Beane, Zohreh Davoudi, and Martin Savage asked what signatures our own universe would carry if it were computed on a lattice of the kind used in present-day calculations of the strong nuclear force. They proposed that the lattice spacing would impose a ceiling on the energies of particles, and that the most energetic cosmic rays would show a preference for the lattice’s directions, a faint texture in an otherwise uniform sky. Whether our world rests on any such lattice is unknown. The proposal shows that a substrate can leave fingerprints on the physics it carries.

76Commitment A world’s reality rests on its…For a designer, those fingerprints are a choice. A physics can be built so that close enough inquiry reveals its floor, or so that the floor stays hidden as far down as inquiry can go. Discovering the floor would teach the inhabitants something true about their world and would take nothing from it. Concealment is a different matter only when it becomes active, when the physics manufactures false evidence instead of simply declining to display its foundations. Opacity of the kind our own physics shows about intrinsic natures deceives no one. A planted history or a painted sky would. Every physics answers, by what it reveals and what it conceals, the question of how much its inhabitants can learn about where their world comes from, and so the designer cannot avoid answering that question either.

77Four objections are strong enough to require a direct answer.

78A designed reality exists only so long as its substrate runs and its keepers allow it to run. The primordial cosmos, so far as anyone knows, depends on nothing of the kind. One might conclude that a dependent world is less real than an independent one, in the way a reflection is less real than the face it reflects, since the reflection vanishes when the face turns away.

79The analogy fails at the point that matters. A reflection depends on the face for its content as well as its existence, and it has no causal life of its own. The carpenter’s world depends on its substrate for its continuation alone. Its contents arise from its own laws, and its events produce one another. The real question is whether dependence for continuation diminishes reality, and the long tradition of Western theology answers it with unusual clarity. Thomas Aquinas held in the Summa Theologica, written between 1265 and 1274, that every created thing depends on God for each moment of its persistence as well as for its coming into being, so that if God withdrew his sustaining action, all things would fall back into nothing. Descartes argued in the Third Meditation that the power needed to preserve a thing from one moment to the next is the same power that would be needed to create it anew. For both thinkers, the world existed only through a continuous act of sustaining, and neither concluded that the world was therefore counterfeit. They held that it was wholly real and wholly dependent, and they saw no tension between the two.

80Dependence bears on how secure a world is and leaves its reality untouched. A child depends on her parents for food and shelter, a patient on a ventilator depends on the machine for every breath, and no one supposes either of them is less real. Nor does anyone know that our own world depends on something. Theologians have claimed for millennia to know it, along with a great deal about the will that does the sustaining, and in all that time they have produced no evidence that physics could recognize. Aquinas and Descartes analyzed the concept of dependence with real care. Their error, and the error of the tradition they served, lay in claiming to know that the concept applied and what the being on its far side wanted. Physics describes how the cosmos behaves and stays silent about whether anything sustains it. What dependence changes is the moral situation of everyone involved, since whoever controls the substrate holds the power to end every life in the world it carries, all at once and without warning.

81Inhabitants who do not know that their world runs on a substrate believe a great many things about it, and one might conclude that they are deceived about all of them. Chalmers answered this objection in “The Matrix as Metaphysics” (2003). A being that has lived all its life in a computed world holds true beliefs about its hands and tables, because its words refer to the hands and tables of its world. The hypothesis that it lives in such a world is a hypothesis about what underlies its physics, comparable to the hypothesis that matter consists of quantum fields. Such a being lacks one truth about its foundations, as we lack the corresponding truth about ours, and a native philosopher who declares her particles the bottom of everything is mistaken in the way our ancestors were mistaken about indivisible atoms. Ignorance of this kind becomes deception only when a designer builds falsehoods into the world, as in the case of the painted stars considered earlier. Whether natives should be able to discover their origins is then a question about the designer’s conduct, and it leaves the reality of their world untouched.

82Searle’s biological naturalism holds that consciousness depends on specific causal powers of biological brains, which running a program does not by itself reproduce. If Searle is right, the natives of a designed reality might be empty in the way described earlier, behaving as subjects while experiencing nothing. The objection therefore belongs to the question of whether a designed world is inhabited, which the precautionary rule already addresses, and the structural account stands either way. A follower of Searle can grant that fires in the carpenter’s world burn its houses while doubting that anyone there feels the heat. Whether a brain built from that world’s matter would have the powers Searle requires turns on whether those powers belong to the pattern of processes in a brain or to something about our particular matter that the pattern leaves out, which is the very point he and his opponents dispute.

83Suppose two copies of a designed world were run in lockstep on separate machines, every state of one identical to the corresponding state of the other. One may ask whether there would then be two realities or one, and whether every pain in that world would be felt twice or felt once and realized twice. Nick Bostrom argued in “Quantity of Experience” (2006) that duplicating a brain duplicates its experiences, so that two identical runs contain twice as much experience as one. If he is right, running a second copy of a world that contains suffering doubles the suffering, and running a second copy of a world full of joy doubles the joy. Related puzzles multiply quickly. A world might be halted for a year of substrate time and then resumed, and its inhabitants would notice nothing, since their experience would continue as though uninterrupted. From inside, a pause and an ending are indistinguishable until the moment of resumption, which in an ending never arrives. A world might also be copied at some moment and the copies allowed to diverge, producing two histories whose natives remember the same past, and one may ask which of them is the continuation of the original.

84Commitment Once two runs of one world…These puzzles are less objections to the account than questions it makes precise, and the lockstep copy completes a series that began with the replayed recording. A replayed recording passes through the states of a world without those states producing one another. Maudlin’s Olympia keeps its causal machinery but leaves nearly all of it idle during the run. A lockstep copy has everything, with its states producing their successors and its counterfactual structure fully in place, so it is a second running world and no playback. Because a reality is defined by its causal organization instead of its location, two lockstep copies are two complete instances of a single pattern, each bearing every mark. The difficulty that Maudlin’s case raised for the replay returns here in a sharper form, since nothing structural is missing from either copy. Whether two instances of a pattern of experience amount to two experiences is a question about consciousness, which the structural account cannot settle by itself. The account does settle one thing. Once two copies diverge, even by a single event, each contains subjects with histories the other lacks, and nothing done to the inhabitants of one can be excused by the existence of the other.

85The name “primordial cosmos” identifies our universe by its position. It stands first in the sequence of realities that might be built from within it, as the world in which the first substrates will be assembled and the first designers will sit before them. The name makes no claim that nothing lies beneath our world, and the claim would be difficult to defend.

86In “Are You Living in a Computer Simulation?” (2003), Bostrom argued that at least one of three propositions must be true. Nearly all civilizations at our stage of development might go extinct before acquiring the capacity to compute minds like ours in detail. Alternatively, nearly all civilizations that acquire the capacity might choose not to use it to compute many histories of their ancestors. If neither of these holds, then computed minds with experiences like ours vastly outnumber uncomputed ones, and we are almost certainly among the computed. The argument has been contested on many fronts, including its assumptions about which observers belong in the relevant comparison class and about whether consciousness can be computed at all, and it has not been refuted to general satisfaction. If its third proposition is true, the primordial cosmos is primordial only relative to us, the first world in our own lineage of realities and perhaps one of many worlds in someone else’s.

87The same marks protect our world along with every designed one. Should our cosmos turn out to run on a substrate we cannot see, it would lose nothing in reality by that discovery. Its tables would still resist, its stars persist, and its people meet in a common world whose events answer their choices, and we would still be made of its matter. Every grief in its history would remain real grief, every kindness a real kindness, and every life that ended here a real life that really ended. The account that establishes the third table as a table is the same account that keeps our own tables from being called fakes. Anyone who held that computed worlds are unreal would be committed to a strange conclusion if Bostrom’s third proposition were ever confirmed, namely that their own children had never existed. No one would accept that conclusion, and its unacceptability counts against the premise that leads to it.

88The primordial cosmos is therefore best understood as one member of the family of worlds fit to be lived in, the first we know of and so far our only home. Its standing in that family rests on the marks it bears, which it shares with every world that bears them, and owes nothing to the accident of having come first. Human thought has made a similar adjustment before. Copernicus moved the Earth from the center of everything to an orbit around the sun, and later astronomy made the sun one star among hundreds of billions in one galaxy among many. Each step took something away from the sense that our place was the measure of all places, and each left the Earth exactly as solid underfoot as it had been. The primordial cosmos may be due for the same adjustment. It would stop being the definition of reality and become the eldest member of that family, and it would remain, for those who live in it, every bit as real as before.

89These distinctions yield a vocabulary for the work of designing realities. The primordial cosmos is our universe, first in the sequence of realities that may be built from within it. A designed reality is one whose laws and initial conditions were chosen by a designer, and a computationally sustained reality is one whose laws are carried out by a computational substrate, the first term concerning where a world’s laws came from and the second the medium that carries them. The substrate is that medium, the system on which a reality’s physics runs, and for the primordial cosmos it is unknown. A reality’s physics is the set of laws it obeys together with the entities those laws govern, as distinct from the substrate beneath them, and a level is a layer of organization at which things act on one another by their own laws.

90A reality that bears the capacities for publicity and participation as well as the three marks of reality is fit to be lived in, and the subjects who actually live in one are its inhabitants. Natives have never lived anywhere else, whether they arose there or were built there, and immigrants came to it from another reality. Both live in bodies of its matter or, where a world allows it, without one. Visitors experience a world through an interface while their minds run elsewhere, and they are not its inhabitants.

91Commitment A word for a use, such…The words “model” and “simulation” stay in this vocabulary as names for a use, since a system is a model whenever someone reads its states as standing for a target elsewhere, whatever its own structure may be. They name real and necessary tools. A designer will run simplified versions of a proposed physics long before building a world, to learn whether stars will form, whether chemistry will be stable, and whether some hidden instability lies in wait. Each such run is a model in the sense that matters and also, at its own level, a small reality, usually far too coarse for anyone to live in. The danger described in connection with Borges’s map lies in the distance between those two descriptions. A word for a use says nothing about whether anyone is home, and it can outlast the emptiness it once implied, so that a team goes on pausing, editing, and discarding its runs long after they have begun to contain someone. The question of what a run’s level now holds has to be asked again each time the run grows richer, and these terms are meant partly as a safeguard that keeps it from going unasked.

92The third table, then, is a table. The medium beneath it subtracts nothing from its solidity, or from the reality of the woman whose cup it holds and the friend who wipes away the ring. So far as anyone knows, her world differs from the primordial cosmos in only two ways. Someone chose its laws, and its continued existence depends on a machine and on whoever keeps the machine running.

93Commitment Dependence on a keeper is the…That dependence leaves the world’s reality untouched, though it hands the keeper of a substrate a power that no one in the primordial cosmos has ever held over anyone. That power includes the ability to halt every life in a world at once, to alter the laws of that world while its inhabitants sleep, to watch everything that happens in it, and to end it without leaving any trace that anyone inside could detect. The inhabitants would have no recourse against such a power and might never learn that it existed. A fact that is harmless to a world’s reality becomes, once it is placed in someone’s hands, the most serious political fact about that world.

94The woman at the third table knows none of this. She finishes her tea and sets the cup down, and the table holds it, as tables do. Her friend laughs at something she has said. The light from the window has moved a little way across the floor while they talked, and the evening is coming on. The table beneath her hands is solid in every sense Stebbing gave the word. It is made of something she cannot see, and far beneath anything she could ever reach, it rests on a machine that someone has to keep running.