Same difference: how Rawls’ arguments for the difference principle motivate sufficientarianism

Rawls famously proposed the difference principle as a principle of distributive justice. 

The difference principle: social and economic inequalities are just if and only if they’re part of an arrangement that maximally benefits the worst-off members of society.

In other words, inequality between the richest and poorest is only just if it makes the poorest as well-off as they possibly can be, but not simply because it makes the richest even richer. If implemented, this would have dramatic implications. A billionaire could not justify getting another billion dollars simply by giving some money away to the poorest. They would have to ensure the poorest were as well-off as they possibly could be before helping themselves to more.

Why the difference principle?

Some background is necessary to explain the motivation for the difference principle. It is the second of Rawls’ two principles of justice, which also include:

(1) Equal liberties; and

(2a) Fair equality of opportunity (the difference principle is 2b).

Rawls argued it would be rational for parties reasoning from behind a ‘veil of ignorance’, which deprived them of knowledge of what their lot in life would be, to accept these principles as the standards regulating society. Rawls is not just saying that these are good moral principles on which to base society. He is saying it would be rational – in the best interests – of each party to agree to these principles. 

Rawls could have included a different principle of distributive justice, for instance a sufficientarian principle, in his two principles of justice. So why the difference principle? Rawls provides three separate justifications. 

First, Rawls claims that a person wouldn’t care much about they would gain above the minimum level guaranteed by the difference principle.

Second, Rawls posits that the risks of accepting a distributive principle would simply too high for anyone to rationally accept. For instance, a utilitarian principle could in theory leave the worst-off person with nothing if this somehow maximised the aggregate amount.

The third justification needs a bit more explanation. Rawls invokes Kant’s second formulation of the categorical imperative. This holds that it is morally wrong to treat a person merely as a means to an end; they should always be treated as an end in themselves. Only the difference principle, Rawls claims, would treat persons as ends in themselves when distributing social and economic benefits. By contrast, other principles, like a utilitarian principle, would be ‘prepared to impose on those already less favoured still lower prospects of life for the sake of the higher expectations of others’. 

The problem with this justification is that it involves a moral consideration. Yet Rawls explicitly states that moral considerations are not available to parties in the original position – they are motivated only by rational self-interest. However, the justification could be reconciled with this feature of the original position if we accept that treating persons as ends and not mere means is essential to securing the social bases of self-respect. By self-respect, Rawls means a person’s feeling of self-esteem and confidence that their life’s plans are worth carrying out. Importantly, it would be in the rational self-interest of a person, whoever they are, to desire the social bases of self-respect.

Assuming this is correct, Rawls might appear to have provided solid – although indirect – grounds for why parties reasoning behind a veil of ignorance would choose the difference principle over others. If they chose another, they would risk weakening the social bases of self-respect. The person already worst-off in a distribution could be made even worse-off simply because this benefitted somebody already better-off. The worst-off person would thus be treated as a mere means, suggesting that they are not important at all and thereby undermining their self-respect. Significantly, this would be true even for a sufficientarian principle. Even if the worst-off person’s standard of living was already above a given level, they could still be treated as a mere means to the end of making somebody else better-off.

Why not (only) the difference principle?

I find all of Rawls’ justifications of the difference principle unconvincing. 

The first and second are implausible. It seems perfectly reasonable for a person to desire more than the minimum level guaranteed by the difference principle, so they could go on a nice holiday, buy luxury goods, etc. In addition, it does not seem irrational to make a calculated gamble to secure an amount greater than the minimum level.

The third justification is ingenious but flawed. This is so even if one grants Rawls’ assumption that it would be rational for parties reasoning behind a veil of ignorance to secure the social bases of self-respect.

The reason is that it is unclear that Kantian considerations about treating persons only as ends and not mere means would be relevant to parties in the original position. Importantly, this is not because such Kantian considerations are moral ones; though the parties wouldn’t have moral considerations per se, they might be relevant in that they could be closely connected with securing the social bases of self-respect. Rather, it’s because parties in the original position would be selecting a principle to regulate the distribution of socio-economic positions, which would be anonymous (i.e. unattached to persons).

Kant’s maxim that one should treat persons only as ends and not mere means is grounded in the claim that, as rational beings, persons have intrinsic dignity, which is inviolable. However, since socio-economic positions do not have intrinsic dignity, it is not clear (assuming Kant’s maxim is right) that one should treat them only as ends. Consequently, it is not clear that a person’s self-respect could be undermined simply because they occupy a socio-economic position that has been treated as a mere means. 

One might respond that, since it is inevitable that all socio-economic positions will be attached to persons, treating a position as a mere means is, effectively, to treat the person ultimately occupying that position as a mere means. But, as Rawls stipulates, parties in the original position do not know the probabilities that any particular person will be attached to any particular position. 

This means that, before persons are attached to positions, no person could reasonably claim they have been treated as a mere means. Moreover, provided the distributive principle chosen by parties in the original position is in the ex ante equal interest of everyone, I’d suggest that nobody, even after they have been attached to a particular position, could reasonably claim to have been treated as a mere means.

All this means that Rawls’ third justification fails to show why parties in the original position would choose the difference principle over another that also secured the social bases of self-respect. In other words, it underdetermines the difference principle.

Why sufficientarianism?

If parties in the original position could choose a different distributive principle, what should it be? A plausible contender, I’d suggest, is sufficientarianism.

Sufficientarianism passes both Rawls’ first and second tests. It would give parties in the original position the assurance that they would not end up too badly, along with the option to make a calculated gamble to gain more than the guaranteed minimum level.

It could also secure the social bases of self-respect. Let us assume that a necessary and sufficient condition for securing the social bases of self-respect is that everyone has enough resources (both economic and social) to participate in society – for instance, they can attend and give their views at public meetings. (As I’ll argue in the second blog post, this is a plausible interpretation of what the social bases of self-respect consist of). Provided it is specified in the right way, sufficientarianism would ensure this is the case. In other words, it passes Rawl’s third test too.


Rawls’ three justifications of the difference principle fail to show why parties in the original position would choose it over sufficientarianism. The table below sums this up.

Satisfaction with the minimum levelA person wouldn’t care much about what they would gain above the minimum level guaranteed by the difference principle.Implausible – seems perfectly reasonable to want more than the minimum level.
Less riskThe risks of accepting another distributive principle would simply be too high for anyone to rationally accept.Not necessarily true – for instance if the alternative also guaranteed a minimum standard of living.
Secures the social bases of self-respectOnly the difference principle would secure the social bases of self-respect by treating persons as ends and not mere means in the structure of society. Not true – another distributive principle, such as sufficientarianism, could as well. 

If you buy Rawls’ reasoning about how to identify the right theory of distributive justice, this makes sufficientarianism a plausible contender.

In the next blog in this series, I’ll build on Rawls’ ideas about self-respect to propose a novel ‘currency’ of sufficientarianism. Refer to the introductory post to the series here.

Enough, already! Why Rawlsians could be Sufficientarians

In this series of blog posts, Rob makes three arguments related to sufficientarianism, which is a theory of distributive justice – an area of moral philosophy concerned with the question of how resources should be distributed in a society.


Sufficientarianism roughly holds that a just distribution is one in which all members of society have at least enough to live a dignified life (however that might be defined). Beyond this point, how resources are distributed becomes much less important from the standpoint of justice. This means, from the perspective of justice, it would be okay for there to be billionaires driving round in Ferraris or sailing round in luxury yachts (or whatever they like to spend their money on), provided everyone else has enough for a quality-of-life above a given level. 

I’m not going to rehash arguments for and against sufficientarianism in this series (there’s a good review article on this here). Instead, I’m going to present three distinct but related arguments about sufficientarianism. 

I’ll explain how the arguments made by the philosopher John Rawls for a different principle of distributive justice, the difference principle, also support sufficientarianism. While this doesn’t show sufficientarianism is the correct theory of distributive justice, it does – if you buy Rawls’ reasoning – make it a plausible contender.

  • The right currency of sufficientarianism is self-respect

I’ll propose a novel ‘currency’ of sufficientarianism. Currency here means the metric that can be used to quantify how much everyone needs to live a dignified life. Inspired by Rawls, I propose this should be enough to secure self-respect for each individual. Everyone should have enough to feel confident in their abilities to carry out their life’s plans.

  • Sufficientarianism is a kind of limitarianism

I’ll defend sufficientarianism from a powerful objection. This claims that sufficientarianism is indirectly self-defeating, since it would allow individuals to amass such large amounts of wealth and influence that they could distort society and prevent a sufficientarian distribution from being realised in the first place. I point out that, with more sophisticated currencies than money or welfare, this is not necessarily true: sufficientarianism imposes an implicit limit on the amount of wealth and influence individuals can amass. 

I’ll conclude that these three arguments motivate a distinctive, Rawlsian-inspired interpretation of sufficientarianism. In other words, they show why Rawlsians could be sufficientarians.

Thought Experiments

Thought experiments are a key tool of analytic philosophy. They involve considering a real-world or imaginary situation which provokes a powerful moral or evaluative judgement. These judgements can then be used to test whether a philosophical theory is right or wrong, much as scientific experiments are used to test whether scientific theories are true or false. If a philosophical theory and our judgements conflict, then that could count as evidence against the theory.

Not everyone agrees on the validity of evidence gathered using thought experiments. Some argue that, because they abstract away from reality (though, as noted above, they can involve real-world situations), thought experiments ignore the emotions, relationships and every-day experiences that shape our actual decision-making. Yet these things, critics claim, are philosophically relevant. More straightforwardly, what happens if two people respond differently to the same situation? Is one correct and one incorrect, as we would conclude in the case of a scientific experiment?

These are challenging questions. Something it might help to bear in mind is the distinction philosopher Derek Parfit drew between ‘deep’ and ‘merely technical’ impossibility. A situation is deeply impossible if it violates the laws of nature. It is merely technically impossible if, however implausible it may be, it is at least consistent with reality. We should be more wary about the usefulness of our judgements in deeply impossible cases. So much about our existence would be different if the laws of nature did not hold that the validity of our judgements here seems dubious. Moreover, while it is possible for two people to respond differently to a thought experiment, this might simply call for refining the situation until one of them changes their mind.

Thought experiments can also be strange, shocking and funny. They give philosophers a chance to show off qualities – creativity and wit – that logical reasoning does not. We hear below from different members of Philosophy Exchange about the thought experiments they find most interesting and what we can learn from them.

Singer’s ‘Drowning Child’ Case – Rob

Imagine you’re walking through a park one day. You approach a small pond, where you’re surprised to see a small child splashing about. There aren’t any adults supervising the child and, as you get closer, you realise they can’t swim. If you don’t jump in and rescue them, they will probably drown. You’re wearing an expensive pair of shoes, which you will ruin if you jump in. What should you do? It seems only a moral monster could say that you shouldn’t rescue the child. Doing so will bring about great benefits (saving a child’s life) for minimal cost (ruining your shoes). Most people would probably even say you have an obligation to rescue the child. You would be morally blameworthy (even if not legally culpable) if you didn’t. Yet, Peter Singer argues, we in rich countries have the opportunity to save a child’s life, but many of us effectively let them drown. By giving relatively small amounts of money to effective charities operating in poor countries, we could save a child (even many children) from dying from preventable causes, such as malaria.

I like Singer’s ‘Drowning Child’ case because it’s so simple yet so compelling. Most objections to it focus on whether the analogy between saving the child and giving to charities operating in poor countries holds. This is important but, in my view, beside the point. Yes, we should scrutinise whether charities really are effective. But this objection argues for seeking better evidence, not that we don’t have an obligation to save the drowning child. The thought experiment leads to a radical conclusion: that we should do more, much more (perhaps giving 10% of our incomes to effective charities, as Singer proposes), to help the poorest people in the world.

If you’d be interested in reading Singer’s argument for yourself, you can download his book ‘The Life You Can Save’ as a free pdf here.

Descartes’ ‘Evil Genius’ Case – Cedar

René Descartes’ famous statement “I think, therefore I am” is the result of a thought experiment in radical doubt. Descartes sought truth. But he endeavored to doubt all of his beliefs because he figured that if he could find a belief that he could not doubt, then he could use that belief as a foundation upon which to reconstruct the rest of his belief system, this time with increased confidence in its truth.

In order to explore the bounds of possible doubt, Descartes imagined that an evil genius was tricking him into believing everything that he thought he knew about the world. Descartes found it possible to think that all of his beliefs about the external world, such as his beliefs about colors and shapes and sounds, were merely the result of the machinations of such an evil genius. Then, however, Descartes turned his thoughts to his own mind. At this point, Descartes realized that if he is doubting, then he must exist, since he must be doing the doubting. Thus, Descartes concluded that whenever he thinks, he exists. This is a belief that Descartes could not doubt.

I strongly feel the force of this argument. I have done this same thought experiment in my own head, and I find that I can admit the possibility of the external world not existing, but I cannot doubt that I exist. However, Descartes then attempted to build upon this result in order to provide his other beliefs. I find these later arguments less airtight. That said, one reason why I appreciate this thought experiment is that it provides a clear path which others can take. Subject all of your beliefs to criticism, and see how well they can withstand that criticism. Once you do this, you will have a much better sense of the epistemic value of your beliefs.

You can read more about Descartes’ adventures in radical doubt in his ‘Meditations on First Philosophy’, published in 1641, and available in many pdf forms online. See, for instance.

Schrödinger’s Cat – April

Imagine a cat, a flask of poison, and a radioactive source placed in a sealed box. If an internal monitor detects radioactivity (i.e. a single atom decaying), the flask will be shattered and release the poison, killing the cat. Without external disturbance, the cat is considered to be simultaneously alive and dead, a state known as the “quantum superposition”. Yet, when one looks in the box, one sees the cat either alive or dead, not both alive and dead. Schrodinger’s Cat is a thought experiment devised by Austrian-Irish physicist Erwin Schrödinger during his discussion with Albert Einstein. It seeks to answer the question of when and why exactly quantum superposition ends.

This seemingly abstruse narrative has much more real-world applications than people expect. Principally, it articulates the problem of measurement. A few months ago, I decided to buy a Fitbit, the smart watch that claims to measure all your vitals, including resting heart rates, sleep quality, blood oxygen level, etc. Like many other health-conscious individuals, I was curious to see how I am doing. A few weeks in, I realised that the watch does much more than quietly monitoring my bodily functions. Seeing my own vitals every single day made me exceptionally aware of the discrepancy between what I hoped them to be and what they actually are, and that simple awareness impacted my vitals. I get anxious when I see my blood oxygen level falls (yes, Covid-19 has made us all paranoid about this very particular standard). The anxiety decreased my sleep quality, which in turn, lowers my blood oxygen level even more. The downward spiral continued till one day I got considerably upset and called my mother, only to hear her laughing on the other end and calling me the “Schrödinger’s Cat”.

I love entertaining myself with Schrödinger’s Cat because it opens my mind up to appreciate that everything in the universe is so interconnected that sometimes even the curiosity of a single human being could alter the reality. It makes me feel so powerful and powerless at the same time: Powerful that I could change the objective by one action; powerless because I cannot control the direction of that change in any sense. The dichotomy that true agency only exists in the face of ultimate uncertainty humbles me.

I stopped using the Fitbit a while ago. I have come to term with the fact that I can never “quietly observe” my own vitals, and instead of inevitably altering them with my curiosity, I’d rather just let them be.

To read more about Schrödinger’s Cat, you could start by reading John Gribbin’s 1985 book, In Search of Schrödinger’s Cat, which explains in layman’s term the logic and implications of the famous thought experiment.

Are Humans Exceptional?

This co-authored post examines arguments for and against ‘human exceptionalism’. Human exceptionalism holds that humans possess a special quality which means they are of greater moral value or regard than non-human animals.

Rob starts by sketching out a dilemma – which he calls the ‘evolutionary dilemma’ – that the theory of evolution poses to human exceptionalism. Rob thinks this dilemma motivates the view that sentience grounds moral value. In response, Eva Read presents an objection to Rob’s argument and explores challenges in measuring sentience, a first step to its usability in practical contexts.

Eva is a MPhil/PhD student in the Foundations of Animal Sentience (ASENT) Project at the LSE, a fascinating interdisciplinary research programme attempting to resolve debates about animal sentience. Check out the project’s webpage here!

Human Exceptionalism and the Evolutionary Dilemma – Rob

1) What is human exceptionalism?

There are different versions of the human exceptionalism thesis. A non-exhaustive list includes:

(a) Distinct capacities A human exceptionalist might appeal to the idea that humans have distinct capacities from animals. Such capacities could include greater intelligence, emotion and sociability, along with the ability to reason and use language. Technically, they must add another claim: that these capacities attribute moral status. I think this view is the most naïve of those I canvas. Its empirical premise is doubtful; biological discoveries are demonstrating that some animals, particularly primates, share the putatively distinct capacities. The core problem, as Darwin put it, is that ‘the difference in mind between man and the higher animals, great as it is, certainly is one of degree and not of kind’.

(b) Moral personhood A more persuasive version of the human exceptionalism thesis is that humans have moral personhood, while animals don’t. Moral personhood is taken to be a quality that isn’t reducible to any specific behaviour. The most famous formulation is Kant’s. Kant argued that moral personhood derives from being a rational agent, which means being able to reason and make autonomous choices. This was so important, Kant thought, because morality only existed through the decision-making of rational agents. For example, concepts like duty and responsibility are only meaningful if we assume that they apply to agents who are able to reason and freely make decisions. Since animals are not rational agents, they do not have moral personhood. Kant even said that ‘so far as animals are concerned, we have no direct duties. Animals are there merely as means to an end. That end is man’.

Contemporary Kantians might not go this far, but do accept a significant distinction in our moral duties to humans and animals. For example, Robert Nozick proposed ‘utilitarianism for animals, Kantianism for people’. By this he meant that, when it comes to animals, the morally right thing could be to maximise their wellbeing, even if doing so involves using one as a mere means to this end (such as by sacrificing one animal to save five). Conversely, we should never use one human as a mere means to the end of maximising wellbeing (ruling out sacrificing one human to save five).

(c) The soul Many religious and philosophical traditions, such as Christianity and Platonism, hold that humans have an immortal soul and capacity to have a relationship with God (or gods), while animals have neither. We see this, for example, in Genesis 1, in which God is said to make humans ‘in his own image’. We can reasonably interpret from their omission that animals are not made in God’s own image. While these religious and philosophical traditions might hold that we do owe some moral duties to animals (for instance, based on the concept of stewardship), these are markedly less than the moral duties we owe to humans.

2) The evolutionary dilemma

The core claim of evolutionary theory is that, through natural selection, complex life evolved from simple life. Humans, correspondingly, evolved from more primitive living things. Assuming this is true, the human exceptionalism thesis faces a dilemma concerning the origins of the special quality that morally distinguishes humans from animals. Its two horns are as follows:


(a) The special quality emerged spontaneously and without precedent, which is implausible; or

(b) It evolved, which undermines the idea that it genuinely does morally distinguish humans from animals in the first place.

Let’s focus on (a) first. Imagine we are present at the birth of the first human. We are about to witness something amazing. Sufficient mutations have occurred in the genes of this new-born baby to tip it from one species into another. Of course, it will be many years until biologists develop the terminology to describe it in this way. However, there is now a difference – even if it’s not perceptible – between the species to which mother and baby belong.

The human exceptionalist might hold that this difference is not only genetic. It is also moral. The baby now has distinct capacities from its mother, which attribute it moral status; or it has moral personhood, while its mother does not; or it has a soul, while its mother does not. These moral differences could then justify treating the baby and its mother in different ways. We might hold that it would be wrong to treat the baby as a mere means to an end, but fine to use the mother in this way. For example, it might be justified to sacrifice the mother to save five people, but not the baby.

This is simply not plausible. The minor genetic differences between mother and baby cannot support such major differences in their moral value or regard. Yet, if the human exceptionalist holds the special quality emerged spontaneously and without precedent, this is what their view entails.

Now consider (b). The human exceptionalist might instead claim that the special quality evolved. While the baby possesses the special quality in its fullness, the mother possesses a proto version of it. The difference in the moral value or regard owed to mother and baby might then be small, perhaps so small as to mean differential treatment of them would not be justified. However, the human exceptionalist is not entitled to this claim without defeating his own thesis. For the special quality that morally distinguishes humans and animals to be genuinely special, a line must be drawn somewhere. It makes no conceptual sense to have, for example, 75% of moral personhood or half a soul.

The evolutionary dilemma fatally undermines the human exceptionalist thesis. The human exceptionalist must either believe something implausible or something self-defeating. They should therefore revise their thesis or adopt a different view.

3) A companions-in-guilt objection

The human exceptionalist could present the following companions-in-guilt objection. Yes, they might say, my thesis is vulnerable to the evolutionary dilemma. However, any view about what it is that attributes moral value or regard (to humans or animals) is vulnerable to a similar problem. This means the evolutionary dilemma that faces the human exceptionalism thesis has less bite.

Assume, for argument’s sake, that what attributes moral value or regard is sentience, by which I roughly mean the ability to feel pleasure and pain. Call this the sentience thesis. This is a natural view for hedonistic utilitarians, like Jeremy Bentham and Peter Singer, who think that the morally right act is the one which maximises pleasure, while minimising pain. A proponent of this view must accept that living things gained sentience at some point of the evolutionary process. We might agree that a single-celled organism isn’t sentient, but what about slightly more complex forms of life? The problem is that whichever point is selected will be somewhat arbitrary. Yet, given that sentience is taken to be the quality that attributes moral value or regard, this seems wrong. A minor genetic difference will be made into a major moral one. We would need to include consideration of the first sentient being in our moral decision-making, but not its immediate predecessor. This seems implausible.

I think this objection can be answered. This is because qualities like sentience are distinct in an important way from the special quality of the human exceptionalism thesis. Consider the difference between categorical and continuous variables in statistics. A categorical variable has a known and fixed number of possible values, for instance the answers ‘yes’ or ‘no’ to the question ‘are you married?’. By contrast, the basic unit of measurement of a continuous variable can, at least in theory, be subdivided without limit. Life expectancy in a given country could be 81 or 58.6 or 67.75. I suggest that the special quality of the human expectionalism thesis is a categorical variable, while sentience is a continuous variable. One living thing can have incrementally greater or lesser capacity to feel pleasure or pain than another.

Assuming this is correct, the problem facing the sentience thesis is much less serious than the evolutionary dilemma facing the human exceptionalism thesis. This is because the minor genetic difference between the first sentient being and its immediate predecessor would not necessarily make a major moral difference. It could instead make a minor moral difference. The ability of the first sentient being to feel a limited amount of pleasure and pain would mean that we would have to factor it into our moral decision-making to some extent. But this could be to a very small extent. This does not seem so implausible.

4) Conclusion I’ve put forward a negative argument against the human exceptionalism thesis, rather than a positive argument in favour of the sentience thesis (or another thesis). Nevertheless, I think that, if successful, my argument does support the view that whatever quality that assigns moral value or regard should be a continuous rather than a categorical variable. In turn, this gives some support to the sentience thesis and, following this, the claim that our moral decision-making should factor in the interests of the animals as well as humans. We might still have good indirect reasons for weighing the moral value of or regard for human life more highly. For example, humans could have a greater capacity for pleasure or pain than animals. Nevertheless, perhaps we should consider becoming vegetarians, or ban factory farming, or try to work out how to reduce wild animal suffering.

Response to Human Exceptionalism and the Evolutionary Dilemma – Eva Read

1) A case for a gradualist move for moral personhood

Reading the above post, it seems that if the gradualist move is available for sentience, then it is not clear why it would not also be available for moral personhood. Accepting that the special quality of moral personhood evolved does not have to be self-defeating for the human exceptionalist. It is not necessarily inconsistent to hold both a) that moral personhood is truly special – in the sense that our species has it and other animals do not, and b) that it was acquired over evolutionary time. We can imagine that whilst Homo sapiens possess moral agency to its full degree, our now-extinct Homo cousins and those who came before them could have possessed it to varying degrees.

This is not so difficult to conceive of when we consider that some levels of ‘proto-morality’ are seen in members of extant animal species today, with behaviours that indicate moral feelings of inequity-aversion, empathy, and altruism. Whilst we might not grant members of these species moral personhood, we might accept that they are moral subjects – individuals whose behaviour can be guided by moral considerations or motivations. The transition from moral subject to moral personhood may well occur on a continuous scale, but the intermediary species between Homo sapiens and our common ancestor with other Hominids are now extinct. This leaves us with a clear categorical distinction between extant species with moral personhood (humans), and those without (other animals).

2) Challenges for the gradualist sentience move

That all being said, my intuitions align with the sentientist. It seems to me that if an individual is capable of consciously experiencing the world – if it feels like something to be that individual – then they are deserving of moral consideration. However, there are significant obstacles to overcome before making the gradualist move in the case of sentience. It seems that rather than existing along a unidimensional scale from less to more experiential capacity, sentience varies along multiple dimensions. The result of this is that it would be a mistake to try to grade different species as more or less sentient than others. A useful analogy here is the concept of some animals being “more evolved” than others, a framework of thinking about evolution stemming from Linnaeus’ laddered hierarchy of evolution. Or, the concept of the Triune brain, with the mammalian brain building upon the paleomammalian brain, which builds upon the reptilian brain. In these examples we underestimated the complexity of the systems in question and enforced an order that is anthropocentric in nature, and not a true reflection of variation across species. The same can be said for sentience. Trying to answer whether a dog is ‘more sentient’ than a salmon makes as little sense as asking which of the two are ‘more evolved’.

3) Dimensions of sentience

Currently, the framework for sentience (see a summary here) considers variation across five dimensions: perceptual and evaluative richness, integration both at one time (unity) and across time (temporality), and selfhood.

Perceptual richness pertains to the detail that an experience is perceived in, and as such relates to the complexity of each sensory system. Each species will have differing levels of perceptual richness for each of their senses. Take, for example, the mantis shrimp who has up to 16 types of photoreceptor cells in their eyes, compared to the three in human eyes. Mantis shrimps are likely to have much higher visual perceptual richness than humans, but we may have higher tactile perceptual richness.

Evaluative richness is the extent to which experiences have an aversive or attractive quality (emotional valence). Emotions have an evolutionary function, with negative states (for example pain, or anxiety) guiding decision-making away from things which are bad for evolutionary fitness, and positive states (for example joy or love) guiding towards those which are beneficial.

Unity refers to the number of conscious perspectives existing within an individual. As humans our experience of the world is highly unified – we have one perspective of the world and how it relates to us. We might assume that within every other individual animal body is one singular agent, but this is not a given. Birds, for example are natural split brains, who lack a corpus callosum which joins the hemispheres of brains in other vertebrates. There are indicators from experiments on pigeons, for example, that there might be two agents acting inside one body.

Temporality relates to whether experience is perceived in a continual flow across time – as is the case for humans. This flow is not a given across other species, instead we could imagine conscious experience happening in snapshots, like in a film montage.

The last dimension is selfhood – whether an individual perceives of itself as one being, separate from but interacting with the external world. This dimension has been of particular interest in the field of animal cognition, with the famous mirror test, for example. Passing the mirror test could indicate one level of selfhood – conceiving of yourself as an individual. Other tests could indicate more complex understanding, for example whether an individual is aware of how much they know, or how confident they should be in their beliefs (metacognition).

4) Implications for moral consideration Let us imagine that we have created a profile for each species with regard to these different dimensions. Trying to find a way to simplify and combine these profiles onto a single sentience scale would be mistaken – how much weight would you give to each dimension? Evaluative richness may have stronger implications for moral consideration than, for example, temporality, but by how much? If we want to use sentience as the basis for grading species on a unidimensional scale of moral consideration, we are left with the challenge of finding an appropriate and reliable way of mapping these complex sentience profiles onto it. Faced with this challenge, perhaps we might do away with a scalar or categorical conception of whether an animal is deserving of moral concern, and instead focus resources into asking other animals what it is that they value and attempting to provide them with such.