Sentience

In 2009 two researchers in Belfast gave small electric shocks to hermit crabs inside their borrowed shells. ^[1] A shocked crab was more likely to abandon its shell, which is what a simple reflex would predict. But the details were not simple. Crabs living in a species of shell they preferred were less likely to leave it after a shock than crabs in shells they liked less. And when shocked crabs that had stayed were later offered a new shell, they approached and switched to it faster.

A reflex does not weigh the quality of a home against the cost of pain, and it does not remember the pain later. The crabs seemed to do both.

Now consider an oyster. It has no brain, only a few pairs of small nerve clusters. It closes its shell when disturbed. Is there anything it is like to be an oyster being opened? Most people have a firm intuition about the crab and no intuition at all about the oyster. That gap between confidence and ignorance is where the science of sentience begins.

What Sentience Means

Two words are often used interchangeably, and it helps to separate them.

Consciousness, in the sense philosophers care about most, means that there is something it is like to be a creature. The phrase comes from Thomas Nagel's 1974 essay "What Is It Like to Be a Bat?". ^[2] A bat navigates by echolocation. We can describe every neuron involved and still not know what hearing the world in echoes feels like from the inside. If there is anything it feels like at all, the bat is conscious.

Sentience is narrower. It is the capacity for experiences that feel good or bad: pain, pleasure, distress, comfort. ^[3] A being could, in principle, have experiences with no positive or negative feeling. Sentience is the part of consciousness that matters most for ethics, because a being that can feel bad is a being that can be harmed from its own point of view.

The problem of other minds

There is no instrument that detects consciousness. You know directly that you have experiences. You infer that other people do because they are built like you and behave like you. That inference, from similarity of body and behaviour, is the only method we have, and it becomes weaker the further we move from ourselves.

David Chalmers called the deeper difficulty the hard problem of consciousness. ^[4] Science can, in principle, explain how the brain discriminates stimuli, reports its states and controls behaviour. Those are the "easy" problems. The hard problem is why any of that processing is accompanied by experience at all. No current theory answers it, and so no theory can simply read off which animals are conscious from their anatomy.

How Science Looks for Sentience

Since consciousness cannot be measured directly, researchers look for indicators that, in humans, go along with feeling pain rather than merely reacting to damage. ^[5]

• Nociceptors: sensory neurons that respond to damaging stimuli. Almost all animals have them, so on their own they prove little.
• Integrated brain processing: signals travel to central structures, not only to local reflex circuits.
• Motivational trade-offs: the animal weighs avoiding harm against other needs, as the hermit crabs did.
• Wound-directed behaviour: rubbing, guarding or limping on an injured part.
• Learned avoidance: the animal avoids places or situations where it was hurt.
• Responses to painkillers: injured animals choose food containing analgesics, or their pain behaviour decreases when given them.

No single marker is decisive. The more of them a species shows, the stronger the case.

What the evidence currently shows

Applied across the animal kingdom, the markers produce a graded picture. The evidence column summarises research findings; the assessment column reflects expert declarations and reviews, which are judgements rather than measurements.

In 2012 a group of neuroscientists signed the Cambridge Declaration on Consciousness, stating that the weight of evidence indicates that humans are not unique in possessing the neurological substrates that generate consciousness, and that all mammals and birds, and many other creatures including octopuses, possess them. ^[6] In 2024 the New York Declaration on Animal Consciousness, signed by several hundred researchers, went further: there is strong support for conscious experience in mammals and birds, and at least a realistic possibility of it in all vertebrates and many invertebrates, including cephalopods, decapod crustaceans and insects. ^[7] It added that ignoring that possibility in decisions affecting those animals is irresponsible.

The law has begun to follow. The United Kingdom's Animal Welfare (Sentience) Act 2022 defines the animals it covers as all vertebrates other than humans, plus cephalopod molluscs and decapod crustaceans. ^[8] The inclusion of octopuses, crabs and lobsters followed a government-commissioned review of the scientific evidence led by the philosopher Jonathan Birch.

Some findings are surprising. In 2022, researchers reported that bumblebees repeatedly rolled small balls in an arena with no food reward, that younger bees did so more, and that bees developed a preference for a chamber associated with the balls. ^[9] The authors argued that this meets the accepted criteria for play in animals.

The oyster

Bivalves are the hardest case. Reviews of pain in molluscs find reasonable evidence of nociception and learned protective behaviour in snails, slugs and especially octopuses, but very little research on bivalves at all. ^[10] Oysters have a decentralised nervous system with no brain, they do not move once settled, and they show few of the behaviours that the markers look for. That may be because they feel nothing. It may be because they have no way of acting on what they feel, or because no one has looked carefully.

Certainty note. The honest answer to "is an oyster conscious?" is that no one knows. The absence of evidence here is largely an absence of research, and the tests were designed for animals that move.

The Ladder That Is Not There

A common view holds that consciousness comes in levels, with humans at the top, apes and dogs below, then fish, insects and finally creatures like oysters. On that view, the lower an animal sits, the less its experiences matter.

In 2020 Birch, Alexandra Schnell and Nicola Clayton argued that this picture is mistaken as a description of minds. ^[3] They proposed that consciousness varies along several independent dimensions:

• perceptual richness: how detailed the experience of the world is;
• evaluative richness: how many kinds of good and bad feelings are possible;
• unity: how well experience is integrated at one moment;
• temporality: how well it is integrated over time;
• selfhood: awareness of oneself as a subject.

An animal can rank high on one dimension and low on another. A bird may have richer colour perception than a human; a bee may perceive polarised light that we do not see at all. There is no single ladder, only a many-dimensional space in which humans happen to be far out on some axes, especially selfhood and reasoning about time.

Why a Simpler Mind Does Not Mean Less Suffering

Humans have long used differences in intelligence to justify treating animals as less important. René Descartes argued in the seventeenth century that animals are complex machines without minds, a view that was used to excuse vivisection without concern for the animals' pain. ^[5] The question of whether animals reason has been used, again and again, as if it settled whether they can be harmed.

The following arguments are philosophical, not experimental. They are presented as the case that this reasoning is wrong, together with the main replies.

Intelligence is the wrong question

Jeremy Bentham put the objection in 1789: "The question is not, Can they reason? nor, Can they talk? but, Can they suffer?" ^[11] The capacity to solve problems and the capacity to feel pain are different capacities. A calculator is very good at arithmetic and, as far as we know, feels nothing. There is no known reason why pain should hurt less in a less intelligent animal.

Some philosophers go further and suggest that simpler minds may suffer in some ways more. A human in pain can understand that it will end, that it has a cause, that a doctor is coming. An animal may experience only the pain itself, with nothing to put it in perspective. This is speculation, but it shows that "less advanced" does not automatically mean "less intense".

We already reject the argument for humans

Newborn babies cannot speak, reason abstractly, or understand their own future. If moral importance depended on those abilities, infants and some people with severe cognitive disabilities would matter less than other humans. Almost no one accepts that conclusion. Philosophers call this the argument from marginal cases: whatever criterion excludes animals tends also to exclude some humans. ^[11]

The argument has a real history. Until the 1980s, it was common for newborns to undergo surgery with little or no anaesthesia, partly because their nervous systems were thought too immature to feel pain. A 1987 review by Kanwaljeet Anand and Paul Hickey in the New England Journal of Medicine assembled the evidence that newborns do feel pain and suffer measurable harm from untreated surgical stress, and practice changed. ^[12] The mistake was exactly the one described here: treating a less developed mind as a mind that could not suffer.

The replies

The opposing positions deserve a fair statement. Some philosophers, following Immanuel Kant, hold that we have duties only towards beings who can reason morally, and that cruelty to animals is wrong mainly because it damages human character. Higher-order theories of consciousness hold that an experience is conscious only when the creature has a thought about it, which might exclude many animals from consciousness altogether. ^[5] Others accept that animals suffer but argue that richer lives, with plans, relationships and a sense of the future, contain more that can be lost, so that killing a person is worse than killing a fish even if their pain counts equally.

That last point is widely accepted and is compatible with the argument above. It concerns the harm of death, not the weight of pain. A being's suffering counts for what it is, whatever else that being can or cannot do. This principle, equal consideration of interests, does not say all animals are equal. It says that equal suffering deserves equal weight. ^[11]

Why It Connects to What We Eat

Almost all the animals humans raise and kill in large numbers, including chickens, pigs, cattle and fish, fall into the groups where the evidence for sentience is strongest or at least realistic. That is why the question of sentience sits underneath debates about diet. The practical side, whether a world without animal farming could feed everyone, is explored in If the World Was Vegan.

The oyster is the interesting exception. Because the evidence for bivalve sentience is so weak, some people who avoid all other animal products still eat oysters and mussels. Others apply a precautionary principle: when in doubt about whether a being can suffer, act as if it can. ^[3] Birch has argued for a version of this approach, in which a being counts as a sentience candidate when there is a realistic possibility of sentience, and decisions should then be proportionate to that possibility. ^[13]

Consciousness and the Soul

Across history, the question "which beings feel?" was usually asked in another vocabulary: which beings have a soul?

The overlap is closer than it first seems. The English word animal comes from the Latin anima, meaning breath or soul: an animal is, literally, a being with a soul. Aristotle, whose work shaped Western thought for two thousand years, used the Greek word psyche for whatever makes a body alive. He described a nutritive soul shared by all living things, a perceptive soul shared by all animals, and a reasoning soul found in humans. ^[14] For Aristotle, animals had souls precisely because they could perceive and feel.

Descartes changed this. He identified the soul with the thinking mind, an immaterial substance that humans have and animals lack. ^[15] Once "soul" meant the capacity for reason, animals lost it, and with it the presumption that they could feel. Other traditions drew the line elsewhere. Jain thought, for example, attributes a soul to every living being, which is the basis of its strict principle of non-harm.

The same question under two names

The following is an interpretation, not a scientific finding.

In many of its uses, "soul" and "consciousness" point at the same thing: the subject that experiences, the "someone at home" who is present in a living person and absent in a corpse. When people ask whether an animal has a soul, and when scientists ask whether it is conscious, both are asking whether there is anyone there to whom things matter.

The words differ in what they carry with them. "Soul" usually implies something immaterial, separable from the body, and often immortal. "Consciousness", in science, is treated as a product of nervous systems that ends when they stop working. Those differences are real, and they cannot be settled by definition. But on the question that matters for how we treat other beings, whether they can be harmed from the inside, the two vocabularies ask the same question. Using the soul to explain what science cannot yet explain has a long and mixed history, discussed in The God of the Gaps.

Theories of Consciousness

Several scientific theories try to say what consciousness is, and they give different answers about animals.

• Global workspace theories hold that information becomes conscious when it is broadcast across the brain, available to many systems at once. Animals with similar broadcasting architectures would be conscious.
• Integrated information theory, proposed by Giulio Tononi in 2004, holds that consciousness corresponds to the amount of information a system integrates as a whole, a quantity measured in the same currency as the information discussed in Entropy. ^[16] On this view, consciousness may come in degrees and may be very widespread.
• Higher-order theories hold that a state is conscious only when the creature represents it to itself, which may restrict consciousness to a few species.

None of these theories is established. Each is consistent with human data and none has been decisively tested against the others. ^[5]

Limitations and Open Problems

• No direct test exists. Every judgement about animal sentience rests on inference from similarity.
• The markers were built around vertebrates. Animals with very different bodies, such as bivalves or insects, may be conscious in ways the tests miss, or may show pain-like behaviour without feeling.
• The hard problem is unsolved. Without a theory of why experience exists, the boundary of sentience cannot be derived, only estimated.
• The ethical arguments are not experiments. Science can say which animals are likely to feel; what follows morally is a separate question.

Someone at Home

The history of this question has one consistent pattern. Each time humans have drawn the circle of beings that can suffer narrowly, whether around humans, around adults, or around those who can reason, the circle has later had to be widened. Newborns were once thought unable to feel surgery. Octopuses and crabs were once excluded from animal welfare law.

What we know is that many animals very unlike us show the signs we rely on to recognise pain in each other. What remains unknown is where experience stops, and whether an oyster lies inside or outside that line. The practical conclusion does not need that line to be found. Suffering does not become smaller because the one who suffers is simpler.

Traditions far from laboratory science reached a similar place from another direction. The Buddhist analysis of Suffering begins with the claim that suffering is a universal feature of sentient life, and meditation is partly training in noticing it. Experiments on the placebo effect show that, even in humans, pain is not a simple signal from the body but an experience built by the brain. And the non-violence that shaped Civil Disobedience was, for Gandhi, inseparable from a refusal to harm animals. Whether a being is conscious is also the question behind the illusion of free will: before we can ask whether someone chooses, we have to ask whether anyone is there.

References

[1] Elwood, R. W., & Appel, M. (2009). “Pain experience in hermit crabs?” Animal Behaviour, 77(5), 1243–1246. https://doi.org/10.1016/j.anbehav.2009.01.028

[2] Nagel, T. (1974). “What Is It Like to Be a Bat?” The Philosophical Review, 83(4), 435–450. https://doi.org/10.2307/2183914

[3] Birch, J., Schnell, A. K., & Clayton, N. S. (2020). “Dimensions of Animal Consciousness.” Trends in Cognitive Sciences, 24(10), 789–801. https://doi.org/10.1016/j.tics.2020.07.007

[4] Chalmers, D. J. (1995). “Facing Up to the Problem of Consciousness.” Journal of Consciousness Studies, 2(3), 200–219. https://consc.net/papers/facing.pdf

[5] Allen, C., & Trestman, M. “Animal Consciousness.” Stanford Encyclopedia of Philosophy. https://plato.stanford.edu/entries/consciousness-animal/

[6] Low, P., et al. (2012). “The Cambridge Declaration on Consciousness.” Francis Crick Memorial Conference, University of Cambridge. https://fcmconference.org/img/CambridgeDeclarationOnConsciousness.pdf

[7] Andrews, K., Birch, J., Sebo, J., et al. (2024). “The New York Declaration on Animal Consciousness.” New York University. https://sites.google.com/nyu.edu/nydeclaration/declaration

[8] UK Parliament. (2022). Animal Welfare (Sentience) Act 2022, c. 22. https://www.legislation.gov.uk/ukpga/2022/22

[9] Galpayage Dona, H. S., Solvi, C., Kowalewska, A., Mäkelä, K., MaBouDi, H., & Chittka, L. (2022). “Do bumble bees play?” Animal Behaviour, 194, 239–251. https://doi.org/10.1016/j.anbehav.2022.08.013

[10] Crook, R. J., & Walters, E. T. (2011). “Nociceptive Behavior and Physiology of Molluscs: Animal Welfare Implications.” ILAR Journal, 52(2), 185–195. https://doi.org/10.1093/ilar.52.2.185

[11] Gruen, L. “The Moral Status of Animals.” Stanford Encyclopedia of Philosophy. https://plato.stanford.edu/entries/moral-animal/

[12] Anand, K. J. S., & Hickey, P. R. (1987). “Pain and Its Effects in the Human Neonate and Fetus.” New England Journal of Medicine, 317(21), 1321–1329. https://doi.org/10.1056/NEJM198711193172105

[13] Birch, J. (2024). The Edge of Sentience: Risk and Precaution in Humans, Other Animals, and AI. Oxford: Oxford University Press (open access). https://doi.org/10.1093/9780191966729.001.0001

[14] Lorenz, H. “Ancient Theories of Soul.” Stanford Encyclopedia of Philosophy. https://plato.stanford.edu/entries/ancient-soul/

[15] Robinson, H. “Dualism.” Stanford Encyclopedia of Philosophy. https://plato.stanford.edu/entries/dualism/

[16] Tononi, G. (2004). “An information integration theory of consciousness.” BMC Neuroscience, 5, 42. https://doi.org/10.1186/1471-2202-5-42