What is sentience?

Juan Carlos Marvizon, Ph.D., Adjunct Professor, David Geffen School of Medicine at UCLA

I must confess that I initially disliked the word “sentience”. Maybe it was because for me it carried religious connotations that should not be mixed with science. Or maybe it was because I saw it as a devious way to avoid confronting the issue of animal consciousness, which would involve answering difficult questions like these: Are animals conscious? Are some animals conscious while others are not? If so, how can we tell them apart? If animals are conscious, is their consciousness the same as ours or there is something unique to human consciousness?

Which ones are sentient?         Source: thinglink.com

Wikipedia gives a definition of sentience analogous to the definition of consciousness: “Sentience is the capacity to feel, perceive or experience subjectively. Eighteenth-century philosophers used the concept to distinguish the ability to think (reason) from the ability to feel (sentience). In modern Western philosophy, sentience is the ability to experience sensations (known in philosophy of mind as “qualia”).” In the context of animal welfare, saying that animals are sentient means that they are able to feel pain. However, as I discussed in previous articles (Not Just Intelligence, The Uniqueness of Human Suffering, The Difference Between Pain and Suffering, More Thoughts on Animal Suffering), we cannot say that a living being feels pain if it is not conscious, because how can there be pain if there is no awareness of the pain? After all, in humans the same stimulus that causes pain when we are conscious does not cause pain when we are unconscious. However, since the problem of animal consciousness is so difficult to address, it does not seem to be realistic to wait until we solve it to decide how we should treat animals. The idea of sentience could be a temporary compromise with which to accept that some animals feel pain and should be treated with compassion. In fact, we all have the strong intuition that the animals with which we interact the most (cats, dogs, horses, etc.) are conscious and able to feel pain. Although it is probably true that these pets are conscious, it also seems that their consciousness is different from human consciousness in some important aspects.

As we move down the complexity scale to small mammals, cold-blooded vertebrates, and invertebrates, it becomes more and more difficult to know if they have feelings. Here, our tendency to anthropomorphize (assign human feelings to animals and objects that do not have them) may deceive us. Even simple animals respond with aversive behavior when given stimuli that we would feel as painful. However, this behavior could well be an automatic response created in the absence of any subjective feelings. After all, plants and even microbes also have aversive responses and we do not infer from them that they are feeling pain. Therefore, it seems that some animals are automatons able to generate responses in the absence of consciousness and pain, while others are conscious and do feel pain. How can we tell them apart? Is there a gradient in consciousness and hence in the ability to feel pain in the animal kingdom? If so, how can we assign moral status to these different levels? Could sentience be a useful concept to designate intermediate levels of consciousness?

The question of what is sentience and how to detect it is not merely academic, because a lot of legislation is being built around this concept. “In 1997, the concept of animal sentience was written into the basic law of the European Union. The legally binding protocol annexed to the Treaty of Amsterdam recognizes that animals are ‘sentient beings’, and requires the EU and its member states to ‘pay full regard to the welfare requirements of animals’” (Wikipedia). Are we foolishly building large bodies of legislation around a concept that nobody can define clearly? Or could there be scientific criteria to define if an animal is sentient or not?

The religious origins of the word “sentience”

Many people assume that the word sentience has a scientific meaning, not being aware of its origin in Eastern religions like Hinduism, Jainism and Buddhism. In fact, I became exposed to the word sentience during my practice of Zen Buddhism. In the Zen retreats called sesshins, we intoned chants in which we expressed our commitment to seek salvation from suffering for all sentient beings, which is one of the main tenets of Mahayana Buddhism. The concept of sentience in Buddhism means “capable of suffering”. However, digging a bit deeper its metaphysical origins start to become clear. Buddhism accepts that suffering requires consciousness, but consciousness is considered a spiritual entity that can be transmitted from one being to another in the process of reincarnation, a belief it has in common with Hinduism and Jainism. Hence, if human consciousness can migrate from human to animal, this means that animals can suffer just like humans and deserve the same moral consideration.

This idea seems to have transmigrated to Western philosophy to be reborn as animal rights. However, it had to be recast as based on evolutionary principles because most Westerners would reject the idea of reincarnation. The naturalistic view proposed by science is that the human mind is not an independent entity but the product of the brain and therefore cannot exist independently from it. By the same token, brains differing vastly in the number of neurons and synapses would produce completely different minds, and only the most complex of them are likely to be conscious. In the secular Western democracies, laws about animal welfare cannot be based in Eastern religious beliefs, just like laws against abortion or homosexuality should not be formulated based on Christian dogma. If animal welfare regulations are to be based on the idea of sentience, it has to stand on a firm scientific and rational foundation. We should be careful not to let religious ideas about consciousness and sentience be smuggled into science and politics.

Agency does not imply sentience

As I pointed out above, one source of confusion about animal consciousness is that when we see animals do things we may infer from that they have intentions and therefore are conscious. However, the ability to “do stuff” is common to all living creatures, not just animals, and does not require consciousness. It was called “agency” by Stuart Kauffmann, a scientist who studies complexity as a fundamental property of life. In his books At Home in the Universe and Investigations, Kauffman defines agency as the ability of living beings to generate their own causes through an internal program which includes their metabolism and genetic information. However, this internal program is an automaton, it is not conscious. Even in humans, the basic processes of life take place automatically, and mental processes and consciousness are additional programs that run on top of this basic software of metabolism. Therefore, just like we would not assume that a robot that does stuff is conscious, we cannot not assume consciousness or sentience merely from animal behavior.

The ability to measure pain in animals does not imply sentience

An argument for an animal being sentient may go like this: “scientists are able to measure pain in this animal, therefore this animal is sentient because it can feel pain”. As one of those scientists who measure pain in animals, let me walk you through what these experiments really entail to find out whether they really measure pain or not.

It is common knowledge that pain thresholds can be manipulated, usually by using drugs that increase them (analgesics) in order to alleviate the pain of patients. Other things like stress and excitement can also alter pain levels, sometimes quite dramatically. Scientists have developed a variety of methods to measure pain that are used to develop new analgesic and to study pain pathways. One type is called “reflexive pain measures” because they measure the reflex withdrawal responses of an animal to a noxious stimulus. These techniques do not hurt the animals, because the stimulus is only mildly painful and animal is able to stop it the moment it feels it. One example of these technique is the “von Frey filaments”: nylon filaments calibrated to exert a fixed pressure to the skin when their tip is pressed against it. They measure sensitivity to mechanical pain by testing the amount of pressure that can be applied to the paw before the animal moves it. These von Frey filaments are also used on humans and can be applied to different parts of the body. If we want to measure sensitivity to heat instead of sensitivity to mechanical pressure, we use a device that shines a strong light on the paw of the animal. The time it takes it to withdraw the paw from the beam of light tells us how sensitive it is to heat.

Conditioned place aversion box. Source: Ugo Basile.

But – here is the catch – these methods do not really measure pain, which is a conscious experience. Rather they measure the ability of the body to process and react to a noxious stimulus, something called nociception. Perhaps the animal is not aware of the stimulus and the withdrawal is merely a reflex. After all, automatic, unconscious withdrawal reflexes occur in humans. What if we want to measure if the animal really feels the pain? Other techniques have been developed to do that. One of the most popular is called “conditioned place aversion”. It consists of a box with two compartments painted with different patterns and with different floor texture. The rat or the mouse is placed in one compartment and given the stimulus that we want to know if is painful. Sometime later, the animal is placed again in the box and given access to both compartments. If it avoids the one where it was given the stimulus, we can deduce that it didn’t like it and therefore it must have been painful. This technique is purported to measure the “emotional component of pain”, something that could be interpreted as suffering.

Example: are crabs sentient?

The crab Carcinus maenas, used in the behavior experiments of Magee & Elwood. Source: Flickr.

In a  2013 paper, Magee and Elwood claimed to have shown conditioned place aversion in crabs, which they interpreted as meaning that crabs feel pain. This was hyped by the media, by Science magazine and (of course) by PETA, and considered proof that crabs are sentient. If true, this could have important repercussions beyond stopping us form eating delicious crabs, because crustaceans like crabs and lobsters have nervous systems smaller (100,000 neurons) than insects like flies (250,000 neurons) and cockroaches (1 million neurons). So, if crabs are sentient, we could soon have laws making insecticides and roach motels illegal. But, is this really the case? Strictly speaking, the Magee and Elwood experiment just showed two things: 1) that crabs are able to differentiate between different locations, 2) that they are able to associate some locations with noxious stimuli and thus avoid them. These are things that we would expect to evolve in crabs because they are key for their survival and do not imply that crabs have the subjective experience of pain. Yes, crabs can learn, but even the worm C. elegans is able to learn to avoid noxious stimuli despite having only 302 neurons and 7,500 synapses. In fact, they did not show even this, because in a later paper in 2016 (which received much less attention by the media), Magee and Elwood recanted their previous assertion by recognizing that in fact the crabs “lack discrimination learning between the two shelters” and concluded that their “behavior is only partially consistent with the idea of pain”. However, I would not be surprised if future studies showed conditioned place aversion in some invertebrate, because, in my opinion, conditioned place aversion does not really measure the “emotional component of pain”, just the ability of animals to associate places with noxious stimuli. This type of associative learning does not require a lot of computing power, much less consciousness. Therefore, we do not really have experimental methods that can tell us for sure whether an animal feels pain, we can only know if it has nociception. “Proof of pain is not possible in any animal” (Elwood, 2017).

Do emotions imply sentience?

Source: slideshare.net

Following on the footsteps of Charles Darwin, who speculated that some animals experience emotions, Paul Ekman proposed that many mammals shared six basic emotions: joy, sadness, anger, fear, disgust and surprise. Darwin and Ekman argued that these emotions can be detected as facial expressions and are not learned or cultural, but genetically inherited. Ekman went on to describe other emotions, like guilt, shame, contempt and pride, that are influenced by culture and seem to be uniquely human. Today, scientists accept that mammals experience the six Ekman emotions and that these are linked to specific states of the brain and the body, such as secretion of adrenaline and cortisol by the suprarenal glands during fear and anger. It is also accepted that pain, itch and pleasure are not just sensations but have a distinct emotional component. Then, should we define sentience as the ability to experience positive and negative emotions? In this view, not just pain but negative emotions like fear and sadness could be a source of animal suffering. In fact, current animal welfare practices in scientific research are based on this idea and consider environmental factors that can be detractive to the emotional well-being of the animal. However, it is unclear if animals other than mammals experience the six basic emotions of Ekman. Do snakes feel sad? Do fish get angry? The most we can observe in these animals are stereotypical flight-fight reactions, but we do not know if they are tied to negative emotions like anger and fear. Therefore, although we could infer that mammals and birds are sentient based on their ability to feel emotions, this still leaves us in the dark regarding other animals.

Sentience, happiness and suffering

A more subtle issue is that both suffering and its opposite, happiness, are not emotions but global states of being that are influenced by emotions but also by other factors. Thus, we can be happy even when we experience negative emotions like sadness (for example, when watching a tragic movie), fear (in a roller-coaster or watching a terror movie) and anger (in the exhilaration of a fight). Conversely, positive emotions like pleasure, joy or pride can coexist with an underlying state of suffering caused by dissatisfaction with ourselves or our life, or existential angst. In humans, happiness and suffering are certainly influenced by emotions, but they are more critically dependent on deeper things like self-esteem, feeling loved and appreciated, flourishing or eudaimonia, and finding meaning in life. In humans, happiness and suffering also exist in time, as a continuum anchored in our past experiences and extending into our outlook for the future. In contrast, although animals have memory, their experience is limited to a narrow time interval around the present because they lack extended consciousness and an autobiographical self (see below). All these considerations show that in humans happiness and suffering cannot be separated from the cognitive and cultural elements that are needed for our self-esteem and sense of meaning. These are not present in animals. Hence, animals cannot suffer and be happy in the same way as humans. Perhaps we can say that humans experience deep suffering and deep happiness, while animals experience shallower forms of suffering and happiness that can be called sentience?

Could sentience be considered “consciousness-light”?

According to Wikipedia, “sentience is a minimalistic way of defining consciousness, which otherwise commonly and collectively describes sentience plus other characteristics of the mind”. But then, how are sentience and consciousness different? What are those “other characteristics of the mind”?

Some concepts of consciousness view it as something unitary and indivisible. When consciousness is viewed solely as our subjective experience, it is hard to see how it can be divided into parts: either one is conscious or is not. This concept of consciousness does not seem to leave room for sentience to be considered as a reduced form of consciousness. Quite the opposite: it would lead us to believe that humans and animals are equally conscious. Nevertheless, we know from experience that there are different levels of consciousness, from sleep to the hyper-alertness produced by danger, meditation or some drugs. States of consciousness can also vary qualitatively, from introverted to extroverted, from intellectual to emotional. All this suggests that consciousness may be formed by different elements.

In contrast with the unitary view of consciousness, I have come to think of human consciousness as multilayered, integrating multiple mental functions. In this view, consciousness can function in a sensory mode in which the different sensations entering the brain are selected according to their emotional weight (“salience”) so that some become conscious and others do not. Consciousness can also function in a proactive or “motor” mode in which it directs attention and behavior to achieve a particular goal. For example, this occurs in a cat stalking a mouse or a dog tracking a scent. In humans, the ability of consciousness to direct attention is used in internalized actions that do not seem to exist in animals, for example, when we recall a particular memory, imagine a future or hypothetical scenario, or engage in rational thinking. In turn, this ability to remember the past and imagine the future gives rise to ‘extended consciousness’: our ability to perceive ourselves as beings that exist through time. Human subjective awareness also allows us to know that we have a mind. I think that this ability of the human mind to reflect on its own functioning derives from the “theory of mind”, the uniquely human ability to make mental models of the minds of other persons. By applying theory of mind to our own mind, we construct a narrative about what we are thinking and feeling. Theory of mind applied to our own mind repeatedly over time may be what give raise to extended consciousness and the autobiographical self or the ego: a continuous narrative of who we are. Extended consciousness, knowing that we have a mind, our internal narrative of our own experience and our autobiographical self are uniquely human, because they require cognitive and cultural elements that only happen in the human mind. Things that we know about ourselves, our society and our culture are indispensable to construct narratives about what goes on in our minds right now, about who we were, and about who we will be.

If this multilayer hypothesis of consciousness is true, it would be possible for animals to have sensory and motor consciousness but not other elements of human consciousness like theory of mind, extended consciousness and a sense of self. Thus, animals would be able to filter sensations to give salience to the ones that are important for survival. Like in humans, pain would be given more saliency because it directs attention to body injuries that need to be protected and healed. Another salient sensation is itch, which alert of attacks by insects or parasites that can be fend off by scratching. Many animals feel pain in this sense of a salient sensation that elicits a particular behavior. However, whether pain leads to suffering seems to require elements of consciousness other than a simply ranking of sensations. These include the presence of negative emotions like fear, anger and sadness, which are experienced by mammals and birds. However, like consciousness, human suffering entails cognitive and cultural elements that are absent in animals. For example, we perceive our pain together with the knowledge that we will continue to experience it in the future, that we have experience it in the past, and that it may be experienced by people we love. All this adds extra dimensions to our suffering. In contrast, an animal only experiences pain as something occurring now, without being amplified by any cognitive elements. In this sense, we could talk of a gradation of suffering that mirrors the gradation of consciousness from less complex to more complex animals, and then to humans. Human suffering has a depth and a meaning that no animal suffering has, because human consciousness is qualitatively different from animal sentience.


In conclusion, sentience seems to be a good word to designate our intuition that some animals possess intermediate forms of consciousness between human consciousness and the automatic behavior of the most simple animals. Animals with rudimentary nervous systems formed by less than a million neurons are likely automatons incapable of any form of feeling. They should be given the same moral status as plants. I cannot see any ethical difference between eating a carrot and eating a clam, since they are both not sentient. On the opposite end of the scale, mammals and birds should be considered sentient and capable of experiencing pain. However, my multilayer hypothesis of consciousness is speculative, so deriving a concept of sentience from it is premature at best. Strictly speaking, there is no scientific way to determine if a given animal is able to feel pain (as opposed to having nociception). Moreover, given that pain, distress and suffering are entirely different concepts, we can only say that an animal suffers based on our empathy and our intuition. For the time being, suffering is more a philosophical than a scientific concept. Therefore, sentience has no scientific basis so far and should be used with care when legislating animal welfare.

Copyright 2019, Juan Carlos Marvizon, all right reserved