Every theory so far tells you to weigh interests, respect persons, or act as a good person would, and every one of them assumes a settled answer to the question of whose interests, which persons, and towards whom.
That prior question is moral status: what it takes for a being to count in its own right, rather than mattering only through its effects on those who do. It is the most consequential question in the subject by a wide margin, because a mistake about it is not a mistake about one act but about who is in the moral world at all, and history's largest moral failures have all been failures here rather than failures of calculation.
A distinction clears the ground. A moral agent can be held responsible: they deliberate, and can be praised and blamed. A moral patient is a being to whom things can be owed. The two come apart, and everyone accepts that they do, because infants and severely demented adults are patients without being agents. What is disputed is where the class of patients ends.
Candidate criteria
Four criteria have serious defenders, and each draws a different line.
Species membership. Human beings count, other animals do not. This is the working assumption of most legal systems and most people, and it is rarely defended explicitly, since stated baldly it looks like an appeal to group loyalty of a kind we reject elsewhere.
Rationality or personhood. What matters is being a self-conscious being with plans, a sense of oneself over time, and the capacity to give and follow reasons. This is Kant's position, and he draws the conclusion without flinching: beings whose existence depends on nature and not on reason have only relative worth as means, and duties towards animals are indirect, mattering because cruelty to them coarsens us towards people.
Sentience. What matters is the capacity to suffer and to enjoy, because that is what makes it possible for anything to go well or badly for a being at all. Bentham put it in a footnote in 1789 that is still the standard citation: the question is not whether they can reason, nor whether they can talk, but whether they can suffer.
Having a welfare. Slightly wider: what matters is having a life that can go better or worse from the inside, which may extend to beings with interests but limited affect.
Each criterion has to answer a test the others pass or fail differently, and the sharpest test is an argument that has resisted answering for fifty years.
The argument from marginal cases
Take any capacity proposed as the criterion of moral status, other than mere species membership: rationality, moral agency, language, autonomy, a sense of self over time. Some human beings lack it. Newborn infants lack all of them. Adults with advanced dementia or profound cognitive impairment lack most. And some non-human animals possess the capacity to a greater degree than those humans do.
The argument then runs: if the capacity is what confers status, those humans lack status and may be treated as we treat animals with similar capacities. If they nonetheless have full status, the capacity is not what confers it, and something else is, and whatever that is may well be present in animals too. Either way the standard position, that all humans have full status and no animals have any, is not supported by any capacity-based criterion.
Peter Singer built Animal Liberation in 1975 on this, coining speciesism for the position that species membership by itself confers status, and arguing that it has the same structure as racism: a morally irrelevant group membership doing the work that capacities should do.
Three replies are worth taking seriously. Carl Cohen argued in 1986 that the capacity for moral judgement characterises humans as a kind, and is not a test administered to individuals one at a time, so an impaired human has the status of a being of a rational kind. This is coherent and has a cost: it makes status depend on the statistical properties of a class rather than on anything true of the individual, which is what we normally call prejudice, and it delivers strange verdicts about a hypothetical rational alien from a mostly non-rational species. Others argue from potentiality, that infants will become rational, which does not help with the permanently impaired and raises the question of why a potential property confers an actual status. Others argue from relations, that our obligations arise from the relationships we stand in, and that impaired humans are members of our families and communities in a way that pigs are not. That last is the strongest, and its cost is that it makes status depend on social connection, which historically is exactly how outsiders came to be excluded.
Nobody has produced a criterion that includes every human being and excludes every animal without relying on species membership itself. That is the state of the argument, and it should be reported as such rather than resolved by a preference.
Example. Someone says the difference is that humans belong to a moral community: we can make agreements with each other and animals cannot. Test the criterion.
The criterion is reciprocity, and it is refuted from inside its own logic. Infants cannot make agreements, nor can the severely demented, nor could a person in a persistent vegetative state, yet none of them may be experimented on or eaten. If the reply is that they belong to a species most of whose members can reciprocate, the criterion has quietly become species membership again, and the argument is circular. If the reply is that we extend the protection of the community to those connected to its members, the criterion has become relational, and it must then explain why a stranger with no connection to anyone is protected and a family dog is not. The exercise is not meant to establish that pigs have the status of persons. It is meant to show that the popular criterion collapses under one test, and that a defensible version of the standard view has to be much more carefully built than most people who hold it realise.
Now you. Singer's principle is equal consideration of interests. Does it imply that a dog and a child should be treated the same?
Answer
No, and the misunderstanding is the most common objection to the position. Equal consideration means that a like interest counts equally regardless of whose it is; it does not mean that different beings have the same interests. A child has an interest in education and a dog does not, so there is no question of equal treatment there. A child and a dog both have an interest in not being burned, and the principle says that interest counts the same in each. Singer also holds that death harms beings differently depending on what they lose, so a being with plans and a sense of its own future loses more than one without, which is why he does not treat killing a fish and killing a person as equivalent. The principle is therefore compatible with very unequal treatment, and what it rules out is discounting an interest because of the species of the being that has it.
The scale, and why it changes the arithmetic
Whatever weight one assigns to an animal's interests, the number of animals makes the question large, and the numbers are not in dispute.
The Food and Agriculture Organization's figures put land animals slaughtered for food at roughly 80 billion a year, of which about 90 percent are chickens. Against a human population of about 8 billion, that is around ten land animals killed per person per year, worldwide. Estimates for fish and other aquatic animals run to a trillion or more and are far less certain, since fish are usually recorded by weight rather than counted.
The conditions matter as much as the count, and the standard case concerns broiler chickens because that is where most of the animals are. Selective breeding has reduced the time to slaughter weight from around sixteen weeks in the 1920s to about six weeks, roughly 42 days, while roughly quadrupling the bird's mass. The resulting animal grows faster than its skeleton and cardiovascular system comfortably support, and lameness and heart failure in fast-growing broiler flocks are documented in the veterinary literature rather than being campaigners' claims.
The arithmetic point is structural and holds whatever number you insert. Suppose, conservatively, that you weigh a chicken's suffering at one hundredth of a comparable human's. Then 80 billion animals a year is equivalent, on your own weighting, to 800 million humans a year, which is a tenth of the human population, subjected to conditions you have already conceded are bad. Scale converts a modest per-animal weight into one of the largest items in any global moral accounting, and this is why the topic cannot be dismissed as a matter of taste even by someone who thinks animals matter much less than people. To reach the conclusion that it is unimportant you have to hold that animals matter not slightly less than humans but essentially not at all, which is exactly the position the marginal cases argument attacks.
Is status all or nothing?
The argument so far has treated status as a switch, and there is a serious case that it is a dial.
Mary Anne Warren argued in 1997 for a multi-criterial account, on which several different considerations generate moral standing of different strengths: sentience gives a being a claim not to be made to suffer, personhood adds a claim not to be killed, membership of a social community generates further claims, and so on. David DeGrazia defends a similar graded picture. On such views a mouse counts, a pig counts more, a chimpanzee more again, and a human most, without any of them counting for nothing.
The attraction is that it matches how almost everyone actually reasons, and that it dissolves the marginal cases argument's sting: an impaired human retains the claims that come from sentience and from social membership even without the ones that come from full rational agency. The cost is that a dial is much easier to abuse than a switch. A graded account gives no principled resistance to grading humans against each other, which is precisely the move a switch was invented to block, and defenders have to add that full status is reached by every human being and cannot be lost, which is exactly the unexplained step they set out to avoid.
Law has in practice adopted the graded view. The European Union's Lisbon Treaty of 2009 requires member states to pay full regard to the welfare requirements of animals as sentient beings, and the United Kingdom's Animal Welfare (Sentience) Act of 2022 recognises vertebrates and, following a commissioned review, cephalopods and decapod crustaceans. None of that gives an animal the status of a person, and all of it gives them a standing that property does not have.
Example. A laboratory proposes an experiment causing moderate pain to 200 mice, expected to yield a treatment that will relieve severe pain in some thousands of people. Assess it on a switch account and on a dial account.
On a switch account with the line at personhood, the mice have no standing at all and no justification is required beyond avoiding gratuitous waste, which is a verdict most people, including most researchers, reject in practice. On a switch account with the line at sentience, the mice count in the same currency as the humans, and the calculation is a straight comparison of pain against pain with the numbers heavily favouring the experiment, provided the expected benefit is real rather than hoped for. On a dial account, the mice count with a lower weight, so the experiment is justified but the weight is not zero, and what follows is a set of requirements rather than a permission: use the smallest number that will answer the question, use anaesthesia and analgesia wherever they do not destroy the result, and do not run the study at all unless the question is worth an answer. That set is the "three Rs" framework, replacement, reduction and refinement, formulated by William Russell and Rex Burch in 1959 and now written into research regulation across Europe. The dial account is the one the law has adopted, and this case shows why.
Now you. Does a graded account of status let you avoid the argument from marginal cases?
Answer
It softens it and does not remove it. The graded account can say that a profoundly impaired human retains full protection against being killed or used, on the ground that some other criterion, membership of a community, the relationships they stand in, or simply being a human being, supplies what rational agency would have supplied. That is a real answer, and notice what it concedes: it accepts that the protection does not come from any capacity the individual has, which was the original point of the argument. What the graded view buys is that this concession no longer forces the conclusion about animals, because standing has several sources and different beings draw on different ones. What it costs is the tidy claim that status is grounded in a property of the individual, and everyone who takes this route should be clear that they have paid it.
People who do not exist yet
The other expansion of the circle runs forward in time, and it has a peculiar structure: future people cannot reciprocate, cannot vote, cannot bargain, and their existence and number depend on what we do.
Almost everyone accepts they count for something. Burying toxic waste that will leak in three hundred years is wrong, and it is wrong because of what it will do to whoever is there, not because of anything about us. The disputes are about how much, and they turn on a technical device with enormous consequences.
Discounting applies a factor to future costs and benefits, reducing them by a fixed proportion per year. Part of the standard rate is uncontroversial: money invested grows, and future people will probably be richer, so a pound spent on them buys less welfare. The contested part is the pure rate of time preference, which discounts future welfare simply because it is future. Frank Ramsey called that ethically indefensible in 1928, arising merely from a weakness of the imagination, and most philosophers agree with him.
The numbers show why this is not a technicality. The Stern Review of 2006 used a pure rate of time preference of 0.1 percent per year, chosen to represent only the probability of humanity ceasing to exist. Over a century that leaves a weight of : a benefit to someone a hundred years from now counts for 90 percent of the same benefit today. William Nordhaus criticised this and used a rate closer to 1.5 percent, which over the same century gives , a weight of 23 percent, and over two centuries about 5 percent. The two analyses differ by a factor of four on how much a distant future person counts, and that single parameter, rather than any disagreement about climate science, accounts for most of the gap between their policy recommendations. A choice of discount rate is a moral choice wearing the clothes of a technical assumption, and the same is true in cost-benefit analyses of pensions, infrastructure and nuclear waste.
Example. Compute the weight given to a benefit 100 years from now under a pure time preference of 0.1 percent and of 1.5 percent, and say what the difference means for policy.
At 0.1 percent, the factor is . At 1.5 percent, it is . The ratio is about four, so the same future harm is worth roughly four times as much under Stern's assumption as under Nordhaus's, and the effect compounds: at 200 years the second rate leaves about 5 percent. In practice this decides whether an expensive mitigation programme now is worth it, because the costs fall in the present and the benefits fall decades to centuries out. Notice what has and has not been shown. The calculation does not establish that Stern is right; a defender of the higher rate can argue that it captures genuine uncertainty about whether the benefits will materialise, or that some partiality towards the near is legitimate. It establishes that the disagreement is ethical, that it is located in one parameter, and that anyone who presents a cost-benefit analysis without stating that parameter has hidden their moral premise inside their arithmetic.
Now you. A government adopts a policy of resource depletion that raises living standards now and leaves people in 200 years worse off than a conservation policy would have. But different people will exist under the two policies, since a change of that size alters who meets whom and who is born. Have the future people been harmed?
Answer
Not on the ordinary comparative account of harm, and this is Parfit's non-identity problem. To be harmed is normally to be made worse off than you would otherwise have been, and the people living in the depleted future would not have existed under the conservation policy at all; their alternative is not a better life but no life. Provided their lives are worth living, they cannot complain, and neither can anyone else on their behalf. Yet the policy still seems wrong, which means the intuition is not tracking harm to identifiable individuals. Three responses exist, and each costs something. Adopt an impersonal principle: it is worse to bring about a state of affairs containing less welfare, whoever is in it, which abandons the idea that a wrong must wrong someone and leads directly to the population arithmetic of the earlier lesson on the good. Adopt a rights-based or threshold view: it is wrong to bring people into existence below a decent standard, even if their lives are worth living. Or accept the conclusion. The problem matters practically because almost every long-run policy, on climate, on population, on genetics, changes who exists, so the non-identity structure is the rule rather than an exception.
Where to draw the line
No criterion draws a line that everyone will accept, and the honest thing is to say what each costs.
Draw it at species and you have a rule that matches practice, protects every human unconditionally, and cannot be defended by any property of the individuals it sorts. Draw it at rationality and you have a principled criterion that excludes infants and the profoundly impaired. Draw it at sentience and you have a criterion with a clear rationale and a boundary problem of its own, since it requires deciding about insects, about fish, and eventually about systems that report distress without anyone knowing whether there is anything it is like to be them. Draw it by relationships and you have an account of why family and community generate obligations, which is true, and no protection for anyone outside them.
Two things can be said with confidence. The direction of travel has been one way: every extension of the circle, to other tribes, to other races, to the enslaved, to women, to children, to animals in the small degree so far achieved, has been resisted with arguments of the same shape, that the excluded lack some capacity or fall outside some relationship, and each extension has afterwards looked obvious. That is weak evidence, and it is evidence. And the cost of an error is asymmetric: excluding a being that counts is a catastrophe of a different kind from including one that does not.
The next lesson takes up what happens once the boundary is fixed. Knowing who counts does not tell you how to divide anything between them, and neither adding up welfare nor obeying a rule explains what one person can rightfully demand of another.