Chapter 5. Who Might Be Home

1When the United Kingdom left the European Union, it left behind a commitment in the Lisbon Treaty to regard animals as sentient beings, and the government’s refusal to copy that commitment into British law drew enough criticism that it promised a law of its own. The bill it drafted aimed to make ministers consider how their policies might harm the welfare of animals, which raised at once the question of which animals the duty would cover. Its first draft named every vertebrate and no other creature, and campaigners asked why octopuses, crabs, and lobsters should be left out. In 2020 the Department for Environment, Food and Rural Affairs commissioned LSE Consulting, part of the London School of Economics, to review the evidence that cephalopods and decapods can feel. The work fell to a team led by the philosopher Jonathan Birch, with Charlotte Burn, Alexandra Schnell, Heather Browning, and Andrew Crump. The department made clear from the start that no other invertebrates were under consideration, so the team was asked about two groups of animals and nothing more. Their Review of the Evidence of Sentience in Cephalopod Molluscs and Decapod Crustaceans (2021) drew on more than three hundred scientific studies.

2Every theory of consciousness is disputed, and a policy that rested on one of them would rest on the dispute, so the team built on an older list of criteria for pain and treated each criterion as a physician treats a symptom, as evidence that raises the probability of feeling without proving it. Their list had eight entries, among them receptors that detect injury, trade-offs between a threat and an opportunity for reward, and behavior showing that an injured animal values a substance that relieves its pain. For each group of animals and each criterion the team stated a level of confidence, and they graded the whole body of evidence for a group from very strong down to unknown or unlikely. The evidence for octopuses was very strong and for true crabs strong, while clawed lobsters, crayfish, squid, and cuttlefish showed substantial evidence. Other groups scored lower, and their scores reflected, at least in part, where scientists had chosen to look. For the penaeid shrimps, which had barely been studied, the team had high confidence in a single criterion, that the animals have receptors that detect injury.

3The law could not hold grades of evidence, since the government required a single boundary. The report offered two places to draw it, one taking in every cephalopod and every decapod and the other leaving out the suborder of decapods to which the penaeid shrimps belong. The team recommended the first, partly for a practical reason, since a boundary drawn through taxonomic names that few officials know would confuse a law that every department of government had to understand. On November 19, 2021, the department published the review, and at the same time it tabled an amendment bringing cephalopods and decapods within the bill. The House of Lords adopted the amendment at its report stage, and the Animal Welfare (Sentience) Act received royal assent on April 28, 2022. The Act works through procedure and prohibits nothing. It provides for an Animal Sentience Committee that may report on whether the government has properly considered the ways a policy might harm the welfare of animals as sentient beings, and it requires a minister to answer each report before Parliament within three months. The animals it counts are every vertebrate other than human beings, every cephalopod, and every decapod, and regulations may add other invertebrates.

4Four years after the review, the department promised in its Animal Welfare Strategy for England (2025) to publish guidance on which ways of killing a decapod meet the existing requirements of the law, guidance that would make clear that boiling one alive is not among them. The whole history ran its course without anyone knowing whether a crab feels. A team graded a body of evidence and published its grades with its confidence, a government asked where a single boundary should fall, and Parliament put a doubtful group of animals inside the law while leaving room to extend it. The decision ran ahead of certainty in the direction the evidence pointed, and it drew the boundary around animals whose feelings had barely been studied, so that doubt arising from neglect went toward inclusion. It also kept two roles apart, the grading of evidence and a recommendation, which the review supplied, and the decision to act on them, which fell to ministers and to Parliament. Precaution became law through a procedure that anyone could inspect, and the procedure did more of the work than any theory of consciousness.

5A designer meets the same decision with every creature a machine computes. A fish in a designed lake is carried in full or summarized, its life computed once or left open to being undone, and the machine has to settle which before the fish has lived a day. Every earlier argument about what the machine may spare has leaned on a rule for these cases without saying where the rule’s boundary falls, and the protected region, drawn so far by examples, needs an edge that can be stated and checked. The question is who might be home in a world where no one can yet say for certain who is, and what a designer owes each system while the question stays open. Answering it takes a theory of where experience is to be expected, held as a working hypothesis with a clear account of what depends on it, and a rule for acting under doubt that tells the machine what to compute.

6Hilary Putnam argued in “Psychological Predicates” (1967) against the theory that pain is identical with a particular state of the brain. A defender of that theory, Putnam observed, would need a single physical and chemical state present whenever any creature is in pain, in mammals, in reptiles, and in mollusks such as the octopus, which he took for granted could feel it. Brains so different are unlikely to share any such state, and the hope of finding one grows dimmer with every new kind of creature that might feel. Putnam’s alternative was that pain is a functional state, defined by what it does, by the inputs that produce it, the behavior it produces, and the other states it interacts with. A state defined that way can be realized in very different matter, as one program can run on machines of different construction. That thesis, that minds are multiply realizable, is one of the oldest supports of the working hypothesis, and the octopus Putnam mentioned in passing has since become its best illustration.

7An octopus carries most of its neurons in its arms, and the last ancestor it shares with human beings lived more than twice as long ago as the first dinosaurs, so its nervous system grew on a plan of its own from something far simpler. The evidence that the review graded very strong, that octopuses protect an injury flexibly, learn by association, and value relief from pain when injured, comes from a body that copies none of the structures that carry feeling in a mammal. If octopuses feel, then feeling does not depend on a cortex or on any arrangement peculiar to vertebrates, and the most natural explanation is the one Putnam gave, that what matters is a kind of organization that very different nervous systems can achieve. The case shows less than a designer needs. It establishes that experience is tied to no single anatomy and leaves open which organizations carry experience and how much of their detail matters. Two animals that both feel can differ in a great deal, and nothing in the comparison says which of their differences could have been removed without removing the feeling.

8David Chalmers gave the working hypothesis its strongest argument in “Absent Qualia, Fading Qualia, Dancing Qualia” (1995). He asked his readers to imagine the neurons of a person’s brain replaced one at a time by silicon chips, each built to respond to its inputs exactly as the neuron it replaces did. At every stage the brain’s organization at the grain of its neurons stays what it was, so the person’s behavior stays the same, and so does everything she believes and says about her own experience. If her experience faded as the chips took over, there would come a stage at which she had only a faint shadow of the colors and sounds she had before while insisting, with full confidence and every sign of sincerity, that nothing had changed. Chalmers judged that so complete a gap between a rational being’s experience and its own beliefs about that experience is not credible. In a second version a switch flips a part of the brain back and forth between its neurons and their silicon copy, and if the two differed in experience, her experience would swing between red and blue while she noticed nothing and could notice nothing. He concluded that any two systems with the same fine-grained functional organization have the same experiences, a conclusion he called the principle of organizational invariance.

9The third support is the absence of anything in the chemistry of carbon that contributes to experience in a way another material could not. The parts of a neuron’s work that have been studied, the movement of ions across its membranes and the release of molecules its neighbors detect, matter by what they do, by the timing, strength, and pattern of the signals that result. The absence moves the burden of argument onto anyone who holds that a mind made of other matter and organized in the same way would feel nothing, since such a person must name the property that carbon has and the other matter lacks. Biological theories of consciousness try to name one, and they are among the rivals the working hypothesis has to meet. Until one of them succeeds, the materials of a brain count as one way of meeting conditions that other materials might meet.

10These arguments establish less than they are often taken to establish. The replacement arguments show that experience survives any change that preserves a system’s organization at the grain the replacement preserves, so they compare a brain with its own fine-grained copies and say nothing about a mind whose organization copies no one. A designer who builds a mind from first principles, with an architecture that resembles no brain at any grain, gets no help from them, since there is no original whose experience the new mind could be shown to share. They also leave open how fine the preserved grain must be. Chalmers took the relevant level to be one fine enough to fix a system’s behavioral dispositions and thought the level of neurons likely to suffice in a brain, and if experience depends on organization finer than the neuron, a chip that copies only a neuron’s inputs and outputs preserves the wrong thing. Peter Godfrey-Smith pressed the point in “Mind, Matter, and Metabolism” (2016), arguing that the functional properties that matter to a mind belong to particular grains and include the activity by which living cells maintain themselves. A living part replaced by an artificial controller, he argued, works differently inside and sooner or later behaves differently too, so the premise that the replacement leaves everything the same fails at the grain where the difference lies.

11Commitment Functionalism is the book’s working hypothesis…What survives is a narrower thesis that is still enough to work with. Experience is confined to no single anatomy or material, as the octopus shows, and among systems that share a brain’s organization at the grain where experience depends on physical detail, it is invariant, as the replacement arguments show. The rivals argue that the material or the physical structure of a system matters beyond its organization, and none of them has evidence that settles the matter. On that basis functionalism is the best working hypothesis available, the claim that a system’s experience is fixed by its organization at the level where organization matters, whatever material carries it. A designer has to answer two questions the hypothesis leaves open. One asks whether a designed mind that copies no one could be conscious, and the other asks at what level of a brain’s organization experience depends on physical detail. No one can now demonstrate an answer to either, so the protections that rest on the hypothesis are built to need only a serious chance that it is true, and the conclusions that would fall without it can be named in advance.

12In “Facing Up to the Problem of Consciousness” (1995), Chalmers separated the easy problems of consciousness, which ask how a brain discriminates stimuli, integrates information, reports its own states, and directs attention, from the hard problem, which asks why any of that processing is accompanied by experience at all. The easy problems are hard enough in practice, and they are easy only in the sense that their answers will take a form science knows how to give, a mechanism that performs a function. An answer to the hard problem would have to say why performing the function feels like something, and a mechanism, however complete, seems to leave that question standing. The working hypothesis says where experience is to be expected and leaves the reason it is there unexplained, much as a physician can predict which injuries will hurt without knowing why hurting feels as it does. Every judgment a designer makes that a system feels will rest on evidence about what the system does and how it is organized, and none will rest on the experience itself.

13A working hypothesis is held because action needs one and the evidence favors it, and it stays open to revision. It gives a designer a rule for expecting experience where certain organization is present, a way of reading anatomy and behavior as evidence about that organization, and predictions that would count against it. A copy faithful to the function of some part of a brain that changed the person’s reports of her experience anyway would refute it. It cannot give certainty about any particular case, because the gap the hard problem marks lies between all the evidence anyone can gather and the experience itself. A designer who refused to act until that gap closed would act all the same, since any physics carries some organization or other, and refusing to choose by a hypothesis would leave the choice to chance. The uncertainty the hard problem leaves is the reason precaution exists. If anyone could look past behavior and organization and see experience directly, the protected region could be drawn by inventory, and no line of protection would be needed.

14Patrick Butlin, Robert Long, and their colleagues adopted a hypothesis of the same family for a narrower purpose in “Consciousness in Artificial Intelligence” (2023), where they checked artificial systems against indicators drawn from leading scientific theories of consciousness. They took computational functionalism, the thesis that performing computations of the right kind is necessary and sufficient for consciousness, as a working hypothesis for pragmatic reasons, since unlike its rivals it implies that consciousness in an artificial system is possible in principle. They set integrated information theory aside because it is incompatible with that thesis, and their analysis suggested that no current artificial system is conscious. The working hypothesis defended here rests on more than their pragmatic reason, since it draws on the octopus, the replacement arguments, and the absence of evidence that the material matters. A designer has to choose a physics, and a hypothesis that lets evidence bear on what the physics will hold is the only kind on which the choice can be made responsibly, provided it is held with the confidence its evidence supports and the rest is left to precaution.

15John Searle’s “Minds, Brains, and Programs” (1980) is remembered less for its comparison of a modeled mind with a modeled fire than for a room. A man who knows no Chinese sits inside it with a book of rules written in English, and slips of paper covered in Chinese characters come in through a slot. The rules tell him, by the shapes of the characters alone, which characters to send back, and his answers are good enough that people outside take the room for a fluent speaker. The man understands nothing of the questions or of his answers. Searle concluded that running a program cannot by itself produce understanding, since the man runs the program and understands nothing. To the reply that the whole room understands, the man being only one part of it, Searle answered that the man could memorize the rules and work everything out in his head, and he would still understand no Chinese.

16Ned Block built a larger room out of a nation in “Troubles with Functionalism” (1978). He imagined the government of China arranging for its billion citizens, each with a two-way radio, to realize for an hour the functional organization of a single person’s mind, with each citizen playing the part of one unit in that organization and the whole connected by radio to an artificial body. If functionalism is true, the system would have the person’s mental states, pains among them, for the hour it ran. Block found prima facie doubt that it would have any states with a felt quality at all, and he took the doubt as a sign that functionalism counts too many systems as minds. Both arguments place the right organization at a scale and in a medium where the reader’s imagination finds no one home, and both ask the reader to trust that finding.

17The finding is weaker evidence than it seems. The man in the room is one part of the system, the part that moves symbols, and his failure to understand shows that the part lacks understanding, as a single neuron’s ignorance of English shows nothing about whether a brain can speak it. When he memorizes the rules, the program runs in his head as a system his own understanding does not reach, and his ignorance remains evidence only about him. A room in which a clerk’s hand worked through rules rich enough to pass for fluent Chinese would answer more slowly than anyone could watch, and an intuition formed about a system no one could watch at work is an intuition about a picture of it. The nation invites the same doubt, since a reader who pictures citizens with radios finds nobody behind them, and a reader who pictured a brain at the scale of its molecules would find nobody there either. Chalmers extended his own argument to a population, concluding that an appropriately organized population would have the experiences of the brain whose organization it shared, and he judged intuition an unreliable guide to what is possible in such cases. Searle and Block took their cases to tell against functionalism, and the replies leave that dispute open while showing that intuitions of this kind cannot close it.

18A system can produce the right answers by a route that shares nothing with the organization of a mind, as a table that paired every possible question with a stored answer would, and a figure in a designed world that talks like a friend may be such a system. Behavior settles little about who is home, which is the room’s lesson, so criteria for the protected region have to look past behavior to the organization that produces it. The nation teaches the other half of the lesson. If a nation organized like a brain would feel, then subjects can be built from subjects, and the beings of a designed world who compute minds of their own, or who are organized together into something larger, raise the question of whether a second subject has arisen above them. Where a system reproduces a brain’s fine-grained organization, the room and the nation give little reason to doubt that someone is there, and where a system reaches a brain’s behavior by tables or scripts, they give real reason. Their force falls on credences unevenly, and the rule that turns credences into protection has to register the difference.

19Anil Seth’s “Conscious Artificial Intelligence and Biological Naturalism” (2025) challenges the assumption that computation provides a sufficient basis for consciousness. Seth argues that consciousness depends on the nature of living organisms, a view he calls biological naturalism and states as the claim that consciousness is a property of living systems only, though not necessarily of all of them. On his account the predictive modeling a brain performs is bound up with the regulation of a living body, the work by which an organism keeps itself in being, and in such a system what it is cannot be pulled apart from what it does. A simulation of that work lacks the causal powers and the intrinsic properties of the work itself, so a computer running even a detailed simulation of a brain would not thereby be conscious. Seth concludes that real artificial consciousness is unlikely along the present course of the technology and becomes more plausible as machines become more like brains or more like living things. If his view is right, a creature computed on a conventional machine would feel nothing at any grain, however faithfully its life was carried, unless the machine that carried it was itself alive in the relevant sense.

20Integrated information theory reaches a similar verdict by another road. Giulio Tononi and Christof Koch, in “Consciousness: Here, There and Everywhere?” (2015), hold that consciousness is a fundamental property of physical systems with particular causal properties, measured by how far a system as a whole, beyond its parts taken separately, shapes its own possible past and future states. The theory predicts that consciousness comes in degrees, that it is common among living organisms, and that it can occur in some very simple systems, while feed-forward networks, however complex, have none, and neither do aggregates such as heaps of sand. Tononi and Koch also state that a digital computer, even one whose behavior matched a person’s and even one running a faithful simulation of a human brain, would “experience next to nothing.” The reason lies in the machine. What the theory measures is the cause-and-effect structure of the physical parts, the transistors and their connections, and Tononi and Koch expect a conventional computer to break into many small complexes of low integration, given the narrow connections of digital circuits, whatever program it runs.

21The denial covers every grain. A mind computed on conventional digital hardware gains nothing on the theory by being computed more finely, at the level of molecules instead of neurons, since a finer program still runs on the same transistors, and the theory asks only what the transistors do to one another. It denies experience to an emulated person carried out of the primordial cosmos and to the natives of any designed world, shallow or deep, whose machine is conventional, whatever those beings say and do. It allows experience to a computed mind only where the hardware’s own physical elements are integrated as it requires, a condition on how the machine is built that no program can meet or fail. Tononi and Koch even propose, as one way to test the theory, joining a neuromorphic circuit to a living brain to see whether experience changes when the circuit becomes part of the brain’s main complex and not otherwise. Whatever credence the theory deserves, its denial is categorical, and a designer has to answer it.

22Grain skeptics keep functionalism and move the organization that matters downward, to the metabolic or electrochemical activity of cells that a model of spikes and synaptic weights leaves out. Godfrey-Smith, whose objection to the replacement arguments has already appeared, holds that the activities of living cells, their self-maintenance, their control of their own boundaries, and the countless small interactions among their molecules, belong to the functional profile on which experience depends. He finds no reason to think that artificial systems built another way would have experience relevantly like human experience. If any such view is right, a mind carried at the grain of its neurons and synapses would not be the same mind as the one it copies, and it might be no mind at all, so the near horizon closes for the dying and for minds built in shallow worlds. Deep worlds stay open, since a native made of the world’s own matter at full depth carries whatever finer organization that matter supports, provided the world’s chemistry supports the organization that matters. The skeptics’ doubt falls on grain and leaves computation itself standing, which separates them from Seth and from integrated information theory, whose doubts reach every grain.

23Illusionism, the view Keith Frankish defends in “Illusionism as a Theory of Consciousness” (2016), holds that phenomenal consciousness, the felt quality that experience seems to have, is an illusion. Introspection represents certain states of the brain as having qualitative properties they lack, and what is really there are the states themselves, with properties that dispose introspection to misrepresent them, which Frankish calls quasi-phenomenal. François Kammerer argued in “The Normative Challenge for Illusionist Views of Consciousness” (2019) that the view must say whether value depends on phenomenal consciousness. If it does, and there is no such thing, illusionists face consequences for ethics that may be hard to accept, and if it does not, they owe a substantial account of what value depends on. A foundation that rests value on experience inherits the challenge, since it would seem to lose its ground if experience in the philosopher’s sense turned out to be an illusion.

24The foundation survives the challenge in the form it already takes. Value is value for a subject, a being for whom things can go well or badly from its own point of view. Illusionism grants that such beings exist. They have states that represent their own condition and move them to avoid or seek, and they report those states, with all the confidence of a person describing a pain, as feeling a certain way. If the felt quality those states seem to have is an illusion, the states remain, and so does their place in a life, as a burn drives a creature from the fire and then troubles its sleep. The precautionary rule protects whatever the word feeling picks out, which on illusionism is that structure of states and their representation, and every protection attaches to it unchanged. Illusionism also tends to raise the chance that computed minds have whatever human beings have, since an illusion of that kind is itself a matter of organization, and a machine organized like a brain would represent its states as a brain does. The challenge exposes a temptation the foundation must refuse, the inference from a theory that denies phenomenal properties to the conclusion that nothing matters, which would remove the reason to prefer any world to any other and the reason to prefer one’s own relief to one’s own pain.

25Panpsychism, as Philip Goff defends it in Galileo’s Error (2019), holds that consciousness is a fundamental and ubiquitous feature of the physical world. Physics, he argues, describes only what matter does, its dispositions and structure, which is Bertrand Russell’s point about the silence of physics on the intrinsic nature of what it describes, and panpsychism proposes that the intrinsic nature behind the structure is consciousness. On Goff’s view the most basic constituents of matter have forms of experience unimaginably simpler than any human experience, and complex experience arises from their combination in some systems, though not in rocks or tables, which have experiencing parts and no experience as wholes. If lifeless matter holds experience, every summary of a lifeless region would seem to erase it. Every saving that makes a world affordable would then fall under the protected region’s prohibitions.

26A designed world’s summaries escape the pressure. On the Russellian form of the view that Goff defends, experience belongs to the intrinsic nature of whatever realizes physical structure, and in a designed world what realizes the structure of a lake is the machine. A summary changes which structure the machine computes, replacing many fine states with a few coarse ones, and it leaves the machine’s own matter as it was, with whatever intrinsic nature it has, running one computation in place of another. Nothing panpsychism counts as a subject is ended by the change, since the computed molecules of a summarized lake were, on the view, only structure, and the machine’s own particles go on existing whatever it computes. Panpsychism also denies that aggregates such as lakes and rocks are subjects, so a panpsychist who believes that experience pervades the machine has no reason to believe that summary ends anyone. The credence that summarizing lifeless matter ends a subject is therefore far smaller than the credence anyone gives panpsychism itself.

27Commitment Under uncertainty about which systems are…Precaution has so far reached the argument in pieces. It first appeared as a policy under which whatever might be conscious counted as a possible subject, and it acquired a measure when the asymmetry between the two errors was used to scale care to the probability that a system feels and to what it stands to lose. In the lake it gained an edge that no saving could cross, because of what a creature already living has left to lose, and the region inside that edge was drawn around fish, birds, and insects. Once the claims of subjects were examined, it divided into basic claims that stand equal for every subject and duties of aid that vary with probability and stakes, and the line of protection was named as the threshold that settles which systems are treated as subjects. Read in sequence, these statements can look like a rule that changed its shape, scaled in one place and absolute in another, though each states a part of one rule. That rule can now be given once and in full.

28Every system that might feel is assigned, by a published procedure and usually through an assessment of its kind, a credence that it feels. A system whose credence stands at or above the line of protection is treated as a subject in full. It holds every basic claim, not to be made to suffer and not to be destroyed against its interest, and every protection of the protected region, so that it is computed in full whenever it is computed, computed once, and never summarized or rolled back, and none of these protections varies with its credence. Duties of aid toward any system, at any credence, are scaled to the probability that it feels and to what it stands to lose. Below the line of protection, summary, rollback, selection by early death, and the other acts forbidden above it are weighed against their costs by the same two measures, and only suffering imposed for no purpose is ruled out at every credence. Every credence, the procedure that assigns it, and the line of protection itself are open to review, and any doubt about where a system stands is resolved toward the protected region.

29The protections that hold in full above the line of protection hold for two reasons. The basic claims take no discount for the reasons that made them equal, since they constrain what is done to a being, and carrying a being commits its keeper to continue. The rules of the protected region take a threshold’s form because summary, rollback, a speculative run that is later erased, and destruction are acts that a designer performs on a system or refrains from, and none of them can be done by degrees, since a fish’s life cannot be half summarized or a stretch of it half undone. A rule for acts of that kind under uncertainty must either set a threshold or trade each act against its savings case by case, and for a being treated as a subject that trade is what the protected region forbids. Aid comes in amounts. A machine can spend more on a creature’s behalf or less, compute its surroundings at a finer grain or a coarser one, study it closely or barely, and carry it sooner or later. So duties of aid vary continuously with the chance that the aid reaches someone and with what it gives the one it reaches, whatever the credence. The line of protection therefore rules out for subjects the acts that admit no degree, leaves those acts to weighing below it, and lets aid scale everywhere, which is how a single rule can be absolute in one place and scaled in another. It also refuses a discount for doubt above the threshold, for the reason that refused a discount for status, since a designer who could weaken a being’s protections by making its inner life harder to read would be rewarded for opacity.

30The rule takes its form from the approach to precaution that Jonathan Birch set out in The Edge of Sentience (2024). Birch holds that people ought at least to avoid causing gratuitous suffering to sentient beings, that it is reckless to create risks of suffering for a sentience candidate without asking which precautions would be proportionate to those risks, and that judgments of proportionality should be informed, democratic, and inclusive. He suggests citizens’ panels that ask whether a proposed precaution is permissible in principle, adequate, reasonably necessary, and consistent with how comparable cases are treated, and he asks that their decisions be revisited as evidence changes. A sentience candidate, in his definition, is a system for which there is evidence implying a realistic possibility of sentience that it would be “irresponsible to ignore” in decisions affecting it, evidence rich enough to identify risks to its welfare and to design and assess precautions. A system that falls short of that standard is a priority for investigation when further study could plausibly make it a candidate and human activity affects it in ways that might call for precautions. Birch counts the judgment that a system is a candidate as mixed, partly scientific and partly evaluative, and he requires a substantial base of positive evidence, holding that an inability to rule sentience out is too low a bar. He offers a procedure and says plainly that it is not a decision rule, and when he asks how far a precaution must reduce a risk, he rejects any threshold fixed for every case, observing that no one agrees on precise probabilities at the edge of sentience.

31Commitment Precaution follows a stated rule of…A designer’s machine needs more than a procedure, because it decides, for every creature it computes, whether to carry that creature in full, and it decides continuously and at a scale no panel could follow case by case. The rule keeps Birch’s structure and adds a stated threshold, the line of protection, a probability of feeling at and above which a system is treated as a subject in full, modeled on his sentience candidate. The figure makes the rule checkable, since anyone can ask whether a kind’s credence was assessed by the published procedure and whether it meets the threshold, and it makes every error traceable to the credence that produced it. Deliberation keeps the place Birch gives it, since deciding how much risk of the graver error to accept for the cost of the lesser is a judgment of value that belongs to those who bear the risks and to those who will live under the rule. A stated threshold is also a commitment that can be held against whoever applies it.

32Pure expected value would replace the rule with a single calculation. Every possibility of feeling would count in proportion to its probability and to what is at stake, summed across every system that might be affected, and the designer would choose whatever made the sum largest. A sea holds microbes in numbers no census could reach, panpsychism adds a credence for lifeless matter, and on some theory the machine itself might feel. Under pure expected value those tiny probabilities, multiplied by vast numbers, would dominate every decision, so no summary could be permitted, no lineage could be left to selection, and the dying, whose capacity to feel no one doubts, would be outweighed by speculation about everything else. Hayden Wilkinson defends the principle behind these results in “In Defense of Fanaticism” (2022). He calls fanaticism the judgment that for any modest value that is certain and any probability however small, some value is large enough that a gamble with that small probability of it is better, and he argues that refusing fanaticism costs more than accepting it. A view that refuses it faces, on his account, three choices between two costs, and it must bear at least one cost of each pair. The first pair sets declining some trades of a little certainty for a great gain in what is possible against accepting a series of such trades that leaves it worse off. Another sets inconsistency between decisions at large stakes and small against an absurd sensitivity to tiny differences in probability. The last sets verdicts that depend on remote events nothing one does can affect, events in ancient Egypt among them, against verdicts that depend on uncertainty about those events, which can rank gambles against the way the view knows it would rank them after learning more.

33The precautionary rule takes on some of those costs openly. At the line of protection a small difference in credence makes a large difference in protection, which is the sensitivity to small differences in probability that Wilkinson counts against views like it. The rule accepts the cost for reasons that belong to the work it does. The credences that would dominate under pure expected value are not measurements, since no agreement exists on precise probabilities of feeling, and multiplying an unmeasured number by an enormous one turns an artifact of how theories are counted into a command. The rule must also be public and open to contest, and a rule that any unfalsifiable possibility with enough at stake could capture would hand the keeping of a world to whoever names the largest possibility. Below the line of protection the weighing counts numbers as well as probabilities, so a precaution that spares a billion worms counts for more than one that spares a single worm. A possibility of feeling that rests on nothing beyond the impossibility of ruling it out can justify precautions that cost almost nothing, and no count of the systems that share it can turn it into a claim against beings whose feeling the evidence supports. The ground for that limit is Birch’s reason for requiring positive evidence before candidacy, carried below it, since a bar set at what cannot be ruled out would let in everything.

34A bare threshold of credibility would go wrong in the other direction. Under such a rule everything above a threshold would be protected and everything below it ignored, so the threshold would carry the whole of every decision, and a kind placed just below it by an error of assessment would receive nothing at all. A worm below the threshold could be made to suffer for amusement, and a lineage nearing the threshold would go unwatched until the day it crossed. The scaled care the rule keeps below the line of protection prevents each of these failures. Nothing that might feel is made to suffer for no purpose, precautions that cost little beside the chance and the stake are taken, kinds near the line of protection are studied, and a kind misplaced below it loses part of its care and keeps the rest. The line of protection decides who is treated as a subject, and the care below it decides how everything else that might feel is treated, so that care falls away by degrees on the far side of the threshold.

35Commitment Of the two errors, treating a…The asymmetry between the two errors sets the direction in which the line of protection should err. Treating a populated world as an empty model is the graver error, because it falls on beings able to suffer, so the threshold belongs well below even odds, where a designer accepts carrying many creatures that may feel nothing for the sake of those that do. The lesser error has victims too. On a finite budget, whatever the machine spends carrying in full a creature that feels nothing could have carried someone who does, and while the dying can be carried, the cost of the lesser error falls on people who would otherwise end. A line of protection set as low as doubt reaches would spend lives to protect bare possibilities. The asymmetry tells the designer to set the threshold low, and the budget tells them where low enough stops.

36Commitment No one can yet tell for…The line of protection sits at a credence of one in ten. A system belongs to the protected region when the best available evidence, assessed by the published procedure, gives at least one chance in ten that it feels, and the figure is offered as a reading of the realistic possibility that Birch’s standard for a sentience candidate requires. Birch attached no figure to his standard, and in advising how experts should state probabilities to citizens’ panels he judged unwarranted the government yardstick’s demand for 40 percent before an outcome may be called a realistic possibility, and he doubted that one in twenty is remote. A threshold of one in ten sits within the range those remarks leave open. A chance of one in ten is one that people act on when a doctor or a forecaster names it, and it is low enough to keep the direction the asymmetry sets without spending the budget on every bare possibility. The figure takes the first half of Birch’s standard and sets the second aside. An evidence base rich enough to design precautions is needed for precautions that must be designed, and the protections of the protected region, carrying a creature in full and leaving every stretch of its life in place, need no further knowledge of its welfare to apply.

37The line of protection must sit low enough to keep fish, birds, and insects inside the protected region, since the asymmetry forbids leaving outside any kind whose feeling the evidence leaves seriously open. The evidence for insects, the weakest of the three, includes experiments Birch discusses in which bees trade a richer reward against a hotter feeder when choosing which to approach, and Birch counts adult insects among his sentience candidates. A threshold near even odds would demand that a kind’s feeling be established as likely, which the evidence for insects has not yet done. Kinds whose only case is that feeling cannot be ruled out have to stay below the threshold, among them the plants, fungi, and microbes, whose protection in full would grow with every generation until it had spent the budget and the means that could carry the dying. The threshold must also clear any credence that panpsychism lends to the feeling of inert matter, measured as the credence that summarizing lifeless matter ends a subject, since a line of protection beneath that credence would bring lifeless matter into the protected region and forbid every summary. For the reasons already given, that credence falls far below one in ten even for someone who takes panpsychism seriously. One in ten meets all three conditions.

38At one in ten the fish in a designed lake stay inside the protected region, each carried in full in its halo of finely computed water for as long as it lives, and so do the birds over the lake and the insects on its surface. Kinds near the line of protection can cross it on a small change in the evidence, and the rule resolves doubt about a kind’s placement in favor of the protected region while the doubt lasts. A kind assessed at one in twelve may be carried in full if the assessment itself is uncertain, since the threshold applies to the credence the evidence supports, and an assessment that cannot tell one in twelve from one in eight leaves the question open. Drawing the region generously in this way costs the budget something. The budget’s claims were heard when the figure was set, and they are not heard a second time against each kind at the margin.

39Commitment The line of protection is set…The figure is a judgment of value, how much risk of the graver error to accept in order to limit the cost of the lesser. Birch’s argument that conflicts of value in a democratic society should be settled democratically, since no group can claim expertise in the relative importance of the values in conflict, applies to it fully. In the primordial cosmos the figure belongs to legislatures informed by deliberative bodies such as citizens’ panels. The placement of each kind against the figure is a judgment of evidence, and it belongs to independent review of the kind the British government commissioned in 2020, by scientists and philosophers who publish their criteria, their confidence, and their grades so that anyone can check their work. In a designed world both judgments are written into the keeper’s policy, the published rules by which a keeper protects possible minds wherever no law of physical properties can, since no law of a world may test for experience. The keeper’s policy is fixed before a world begins, open to contest by those it governs or by trustees acting for them, and revisable by a procedure that is itself published, so the figure can move as deliberation changes and a kind’s placement can move as evidence does.

40Commitment Below the line of protection, nothing…Below the line of protection, nothing that might feel is made to suffer for no purpose, a bar that costs nothing to keep and binds at every credence. Summary and selection are permitted, and the designer weighs, kind by kind, the chance that a kind’s members feel and what they stand to lose against what care for them would cost, above all in means that could carry the dying. Every precaution whose cost is small beside that chance and that stake is taken, such as choosing, among the ways a lineage’s selection could be written, one that imposes less of whatever in it might be suffering. Kinds within reach of the line of protection are priorities for investigation in Birch’s sense, studied for the evidence that would move them and watched in the keeper’s policy as their lineages change. Duties of aid toward systems below the line of protection scale with probability and stakes as they do everywhere, and the weighing that sets them counts the many as well as the likely. Each of these forms of care leaves a record in the keeper’s policy, so care below the line of protection is as open to inspection and contest as the protection above it.

41Umāsvāti’s Tattvārtha Sūtra, composed sometime between the second and fifth centuries and translated by Nathmal Tatia as That Which Is (1994), grades living beings by the senses they have. Beings whose bodies are earth, water, fire, or air have touch alone, and so do plants, while worms add taste, ants add smell, bees add sight, and human beings and many other animals add hearing. The vow of nonviolence takes two forms. The ascetic’s great vow renounces violence toward every living being, those with a single sense included. The householder’s small vow, the partial form suited to a life in the world, renounces injury to beings with two senses or more, as the commentaries explain it, and among the householder’s further restraints is abstaining from harm that serves no purpose. The grading of beings by their senses leaves the protection ungraded, since each vow renounces injury to everything inside the circle it draws.

42The tradition is a precedent for two features of the rule that can look incompatible. It draws its circle of protection generously, around beings with a single sense, a circle wider than any a designer could now afford. It grades beings by their capacities while refusing to grade the protection owed to those inside the circle, so that the grounds of protection come in degrees and the protection does not. Its two vows also show a boundary of full protection set by what a way of life can bear, with the ascetic holding the widest circle and the householder a narrower one while still refraining from purposeless harm. A designer’s machine stands closer to the householder than to the ascetic, since it spends from a finite store, and the line of protection marks where full protection ends because the means that could carry the dying are limited. What the vows asked of conduct, a machine has to carry out in computation, in what it carries in full and what it summarizes.

43Commitment Integrated information theory’s denial of experience…Integrated information theory’s denial is a claim about machines that a builder can act on. A mind’s organization, at any grain, can be the same on two machines that differ in how their physical parts are wired, and on the theory only the wiring decides. A designer who wants a computed mind to be conscious on that theory as well as on functionalism can run it on hardware whose own elements are densely and reciprocally connected, so that the physical system itself has the integrated structure the theory requires. Neuromorphic hardware, whose physical circuits are designed to behave like neurons and synapses, is one place to begin, though whether any present chip has the integration the theory requires is an open question, and the test Tononi and Koch proposed presupposes that a circuit of that kind could become part of a conscious complex. The rule a builder can meet is to carry a mind intended as a subject on hardware integrated in the theory’s sense wherever that can be done, and the conditions for consciousness, when the argument reaches them, inherit it. Meeting the rule may cost more than conventional hardware, and whatever it costs is counted against the same means that could carry the dying.

44The theory’s doubt counts all the same where the rule is not met. If the theory is true, a mind computed on conventional digital hardware feels almost nothing, so the probability that such a mind feels is lowered in proportion to the credence the theory deserves. The denial belongs to the theory’s fundamental claims, which Birch distinguishes from its testable claim about where consciousness sits in the human brain and counts as legitimate speculation that cannot yet be called empirically supported, so the credence it deserves is modest. Its standing in the rule is double. It is a condition on the machine, which a builder who can meet it should meet, and it is a reason for doubt about every computed mind that runs on hardware that fails it. The doubt cannot by itself carry a mind organized like a person’s below the line of protection. For a mind carried at a fine enough grain, that would require the theory, together with every other view that denies such a mind experience, to deserve more than nine chances in ten, and no fair reading of a field in which each of those views is contested comes near that. A mind emulated on a conventional computer stays inside the protected region, holding every basic claim and every protection, while the duties of aid it is owed, which scale with the probability that it feels, are reduced by the doubt the theory adds.

45Functionalism leaves open which level of a brain’s organization is the one that suffices for experience. Chalmers’s criterion for the relevant level, one fine enough to fix a system’s behavioral dispositions, splits the question into two parts. One is empirical, how fine a copy of a brain must be to behave exactly as the brain would, and the other is a residue the replacement arguments narrow without removing, whether experience could depend on detail finer than any that behavior reveals. Even the smallest nervous system shows how hard the empirical part is. Birch notes that the nematode worm steers toward an attractive odor by computation inside a single neuron, one part of whose axon tracks the sweep of the head while another tracks the strength of the odor. A full copy of the worm, he concludes, would have to reach below the level of whole neurons to the arrangement of structures within them. The ladder of grains that Anders Sandberg and Nick Bostrom laid out for a human brain, from functional models down to molecules, marks the possible answers, and nobody yet knows on which rung experience stops depending on what lies below.

46The rule against making a deep world’s people shallow rests on that ignorance. A designer who knew the level at which experience stops depending on finer detail could carry each native at that level and save everything beneath it without loss, so the prohibition of the middle path is a prohibition under ignorance, and it would lapse if the ignorance ended. While it lasts, a native carried at a coarser grain than her world’s depth may be the same subject, a different subject with her memories, or no one, and nothing available can tell the cases apart. The line of protection gives the practice no cover even when the coarsened native would still stand above it. The protected region’s rules forbid carrying a protected creature at reduced detail once its life is under way, because the reduction may end the subject who was there and put a stranger, or nobody, in her place, and a high chance that someone remains is no license for a real chance of ending someone. A shallow world built openly faces only the lesser error, since its natives are coarse from the start and either feel at that grain or do not, while the middle path risks the graver one.

47Commitment The account of what makes a…The structural account of what makes a world real holds whatever theory of mind is true, since its marks concern how a world’s events produce one another and none of them mentions experience. The moral conclusions sort less simply, and the sorting decides how much of what is owed to computed beings rests on functionalism. Some conclusions hold on any theory of mind. That value is value for a subject, that every subject holds the same basic claims, and that duties of aid vary with probability and stakes are claims about subjects wherever they are, and they bind in the primordial cosmos whatever turns out to be true of machines, toward its animals as toward its people. The precautionary rule belongs with them, because it is a rule for acting under doubt about who feels and so presupposes no answer to the doubt. The rules against deception hold too, since anyone who visits a world from outside is a subject in the primordial cosmos, and a painted sky deceives her as it would deceive a native.

48A second group of conclusions rests on some theory that allows computed beings to feel. Everything owed to the natives of designed worlds belongs here, along with everything owed to emulated minds and the duty to carry the dying into worlds where they can go on living. These conclusions require only that a computed being’s chance of feeling reach the line of protection, and for a being organized like a person and carried at a fine enough grain, the credence that functionalism alone is true carries it there easily. Wherever these conclusions hold, they hold in full, so a native of a designed world holds every basic claim and every protection whatever its credence above the line of protection. Duties of aid toward computed beings scale with the credence, so the duty to carry the dying, which is a duty of aid, weakens as the credence that a carried mind feels falls, a dependence the argument about rescue has to carry. A built mind that copies no one belongs here too, though the replacement arguments never reached it, so its credence rests from the start on what the leading theories say about its organization.

49Should functionalism fail, or hold only at a grain finer than any emulation reaches, these conclusions change form as long as some view still allows computed subjects. Integrated information theory would confine every protection of computed beings to minds carried on hardware integrated as it requires, and the rule a builder can meet would become a condition on every world. The grain skeptics’ view would close the near horizon for rescue and for minds built in shallow worlds, while deep worlds whose chemistry supports the organization that matters would keep their natives, at a cost that puts them far beyond any machine resembling those that now exist. Biological naturalism would let a computed being feel only on a substrate that is itself alive in the relevant sense, so designed worlds carried by any machine now conceived would hold no one. A built mind that copies no one is the case on which the rivals differ most. On integrated information theory it may feel on the right hardware whatever its organization, the grain skeptics require that its organization reach their grain, and biological naturalism requires that it live.

50If no theory that allows computed subjects survives, designed worlds hold no one. A designer would then build structures that bear every mark of reality and contain nobody, the duty to carry the dying would have nowhere to carry them, and the protections written for natives would govern empty rooms. The structural account would go on saying truly that those worlds are real, the duties of the primordial cosmos would bind as before, and the rules against deception would protect anyone who visits. That outcome is possible, and the precautionary rule has already priced it, since the cost of treating an empty world as populated is the lesser error, paid in means that could have done good elsewhere. The argument proceeds as though computed beings can be subjects because the credence that they can is high and the error of assuming otherwise is the graver one, and every later conclusion that leans on the hypothesis leans on it in one of the ways just sorted.

51Commitment Until the edge of the protected…Until better evidence refines the edge of the protected region, four criteria place a kind at or above the line of protection, and any one of them suffices. The first asks for an integrative brain, or a structure that does the work of one, together with behavior showing that the kind represents what it stands to gain and lose. Trade-offs between a threat and a reward made in prospect, flexible care of an injury, learning beyond habituation and sensitization, and the seeking of relief when hurt are marks of that kind, and the 2021 review graded them all. Kinship is the second. A kind whose nervous system shares the architecture of studied kin that meet the first criterion stands inside the protected region until study shows otherwise, so that doubt arising from neglect goes toward inclusion, which was also the effect when the law took in every decapod. Built systems, whose architecture is on record, fall under a third criterion, which places inside any system that has in full the indicator properties of more than one of the leading scientific theories of consciousness, in the computational terms in which Butlin and his colleagues stated them. A fourth places inside any emulation that reproduces the behavior of a kind already inside, and Birch argues for this step in the case of a complete neuron-by-neuron emulation of an insect, a fish, or a mouse.

52The criteria keep fish, birds, and insects inside, as they must. Fish and birds have brains with well-studied integrative regions, and the behavioral evidence of feeling in both is extensive, while bees meet the first criterion through trade-offs made in prospect and through mushroom bodies, integrative centers linked to learning and memory. Each of the other criteria asks for evidence at least as strong as what places the bees inside. A close relative of a kind inside shares much of that kind’s evidence, a built system that meets several leading theories in full feels if any of them is true and the working hypothesis holds, and an emulation that reproduces a kind’s behavior keeps the organization the replacement arguments protect. Decapods and cephalopods come inside by the first criterion or the second. The penaeid shrimps come inside by kinship, though Birch counts them as priorities for investigation, since their brains hold miniature counterparts of the integrative regions of other decapods, and the work that found those regions cast doubt on their function without showing that they do no work. Kinds without nervous systems, among them plants, fungi, and microbes, meet none of the criteria, and they stand below the line of protection whatever their complexity, since their case for feeling rests on the impossibility of ruling it out. A system that produces a mind’s behavior by a table or a script meets none of them unless its organization does, and since the features that make a figure good company may be the ones that make it a subject, a convincing figure is weighed against the line of protection by its organization, with its behavior as evidence.

53The adult hermaphrodite of the nematode Caenorhabditis elegans has some three hundred neurons, about 185 of them in a ring in its head that researchers increasingly call a brain, and that small brain integrates information from several senses, supports learning and decision, shows coordinated activity across the whole of it, and has a form of sleep. Offered an attractive odor on the far side of a barrier that would dry it out, a worm crosses only when it is hungry enough, which looks like a trade-off. Birch argues that the appearance misleads. The mechanism has been mapped at the level of neurons, and the behavior most likely comes from hunger working through a single inhibitory connection between a neuron that detects the threat and a neuron that registers the worm’s internal state. It is then a direct modulation of one response by another and shows no representation of what the worm stands to gain or lose. He counts nematodes as priorities for investigation, open to reclassification if the evidence changes.

54Under the provisional criteria the nematode stands below the line of protection. It meets the first criterion only if its trade-off shows evaluative representation, which the mapped mechanism gives reason to doubt, it has no studied kin inside the protected region whose nervous systems share its plan, and the other two criteria do not apply to it. Worms in a designed world may be left to summary wherever no one perceives or investigates them more finely, and selection may run among them. Scaled care still governs them. Nothing may make them suffer for no purpose, precautions that cost little are taken on their behalf, and the evidence about them is watched, so that a finding of evaluative representation in a nematode’s brain would move the kind inside at once. Because the worm learns, honesty also governs how it is drawn from a summary, and a worm supplied where a lens turns on the silt must carry no association it never formed, which marks one of the few places where the edge of the protected region and the boundary that honesty sets come apart.

55The criteria are provisional and built to err in one direction. They place a kind inside on a single convincing trade-off made by an integrative brain or on resemblance to a kind already inside, and they place a kind about which nothing is known beside its nearest studied relatives. Better study of animal minds will refine them, as will minds built with architectures no animal shares, and each refinement will move the edge outward whenever evidence warrants. What they decide now is enough to tell a machine what to carry in full on the first day of a world. Everything they place inside is computed whenever it is computed, once and in full, and everything they leave outside is held in a summary that the evidence can still reach.

56At first light the lake in the carpenter’s world is still, and the machine beneath it is doing what the rule requires. Most of the water is a few slow currents and a field of temperatures, cold at the bottom and warming at the surface as the sun clears the ridge, and none of it is computed more finely than anyone needs. Around each fish the water thickens into detail, molecule by molecule at the gills and thinning outward to the distance at which the fish can feel a pressure wave along its flank, and each halo moves with its fish through the reeds. A heron stands in the shallows with its own detail around it, and the first midges over the water are carried in full, each one, for as long as it lives. In the silt below, the worms exist as a density in the summary, a number for each patch of mud. Their kind stands below the line of protection on the evidence now in hand, and the keeper’s policy records what finding would move it.

57Later in the morning someone may kneel at the edge with a glass and turn it on the mud, and the machine will compute the worms beneath the glass as lawful draws from the summary, with no memory of anything that did not happen to them. The silt holds far more of them than the shallows hold fish. On the evidence the chance that any of them feels is smaller than the chance for the midges and far smaller than for the fish, though it is not nothing, and the machine spends on them what that chance and their small stakes are worth. If evidence ever moves their kind across the line of protection, the machine will carry every worm from then on as it carries the fish. The light reaches the far shore first and moves across the water toward the reeds. As it comes the fish begin to rise, each in a halo of water carried molecule by molecule, because a stated rule, applied to evidence anyone can check, found a serious chance that someone is there.