top of page

The Uniqueness of Human Suffering

Unlike animals, humans feel suffering as something that extends beyond the present and that we share with others

Woman standing between desolate and idyllic landscapes
Shutterstock Photo ID: 1887982510 by Jacob_09.

The assumption of animal suffering in the animal liberation movement

Jeremy Bentham, an 18th century utilitarian philosopher, famously asked: “The question is not ‘can they reason?’ or ‘can they talk?’ but ‘can they suffer?’” A utilitarian philosopher of our times, Peter Singer, latched into that question to write his book Animal Liberation. And so the modern animal rights movement was born.

Basically, Peter Singer and many other animal rights activists claim that animals suffer like humans and therefore they should be treated like humans. To put in a more sophisticated way, Peter Singer argues that the moral imperative of equality dictates “equal consideration of interests”, that is, that the interests of all beings receive the same consideration. Animals have an interest in avoiding pain, therefore egalitarianism demands that we respect that interest.

It is argued further that claiming human superiority based on our superior intelligence, our ability to talk, or our culture, is just stacking the cards in our favor because those are the special attributes of our species. By the same token, an elephant may claim moral superiority based on the fact of having a trunk.

The problem of animal suffering

However, the whole argument is based on the claim that animals suffer and, moreover, that they suffer like us.

Singer and the other animal liberationists just assume that they do.

I think this is a faulty assumption that needs to be addressed rigorously, both philosophically and scientifically. However, I understand why animal liberationists take umbrage in it: the whole problem of defining suffering seems intractable at first sight.

Suffering, like happiness and consciousness, belong to a class of concepts that are at the same time abstract and fundamental. Defining them in terms that are non-circular seems nearly impossible.

If you look at dictionary definitions of suffering, you will find that they refer to pain, unpleasantness or perceptions of threat, which are just examples of suffering. This does not represent a problem when the idea of suffering is applied to human beings, because we can get accurate descriptions of their suffering from other people.

However, when we want to apply this concept to animals, we need a clear idea of what we are talking about. Otherwise we risk falling into one of two opposite pitfalls:

  • self-serving callousness - choosing to think that animals do not suffer because this is convenient for us;

  • anthropomorphizing - thinking an animal suffers just because we would suffer in the same circumstances.

The latter feels intuitively true because is based on empathy, a powerful human emotion. However, it is not a rational conclusion.

Just like in the case of happiness and consciousness, the problem of suffering can be studied scientifically. In fact, there are a lot of scientific studies related to suffering because the public demands that scientists find solutions to pain and distress.

Just like with happiness and consciousness, science has not have come up (yet) with a complete description of suffering, but it certainly can tell us a lot of things about it. I think that this information can help us form an educated opinion about whether some particular animal suffers or not.

Agency of living beings

One of the most peculiar properties of life has been called agency. It refers to the fact that living beings seem to be goal-directed: they strive towards keeping themselves alive and making more beings like them. The concept of agency is explained in detail in the books of Stuart Kauffman, a scientist who has done extensive work on the conceptual underpinnings of life and evolution.

However, agency does not imply any form of consciousness or intentionality. It is just something that living beings do automatically because otherwise they wouldn’t be living anymore.

It is important to emphasize this because agency can be confused with the “interest” that Peter Singer talks about. Yes, life perpetuates itself, but that doesn’t mean that living beings are conscious or that they have interests and plans like we do. To think otherwise would be to accept some magical vitalist concept of life that science rejected long ago.

Therefore, we can conclude that plants do not suffer, although they grow, reproduce and even fight their enemies with chemical responses.

Likewise, we should accept that animals that lack a nervous system (like sponges) or that have only a rudimentary nervous system (like worms) do not suffer.

Most people would agree with the idea that not all living beings suffer.

But what about animals with a complex nervous system? Do they suffer?

Pain and suffering

Here we must consider that suffering and pain are often confused, but in fact are not identical. Pain produces suffering, but suffering can be produced by things other than pain, like negative emotional states.

That pain and suffering are not identical is also shown by the fact that people may experience pain and not suffer from it. For example, the pain experienced when practicing some sports, when eating spicy food and by sexual masochists induces positive feelings instead of suffering.

Some drugs called dissociative anesthetics (like ketamine) can selectively turn off the emotional part of pain, leaving intact its sensory component - we are still able to feel the pain, but just don’t care about it.

Given the complexity of this subject, I chose to divide this discussion into two parts: suffering that comes from physical pain and emotional suffering. I will start with the first.


Pain scientists distinguish between three concepts: nociception, pain and suffering. This distinction is recognized even by the Humane Society of the United States, an animal rights organization.

To grasp the idea of nociception, consider the case of a patient who is undergoing surgery under general anesthesia. As the skin and organs of this person are being cut, pain sensory nerves record the damage and send this information to the spinal cord, which continues to the brain. The general anesthetic only stops the conveyance of noxious signals at the cerebral cortex, by disrupting the synchronization of cortical neural networks (Craig, 2010, 2014).

This unconscious processing of noxious information is what we call nociception.

Pain as an emotion

Of course, in an awake person, nociception leads to pain. The key idea here, however, is that the processing of noxious information does not imply the existence of pain. Even if it involves millions of neurons and complex neural pathways.

In fact, nowadays, pain is considered part sensation, part emotion. This is because fundamental aspects of pain are emotional, like its negative valence (we dislike it) and its salience (we cannot avoid paying attention to it).

Is the complexity of a nervous system key to determine the ability to suffer?

Therefore, pain requires a fairly complex nervous system capable of turning sensations into emotions.

Based on this idea, I think is reasonable to infer that animals that lack a nervous system of enough complexity do not feel pain. They just have nociception.

Behavior consisting in avoiding a noxious stimulus should not necessarily be taken as an sign of pain. Because avoiding physical damage is crucial for survival, avoidant behavior can be found even in the simplest animals. Even plants and microbes react to noxious stimuli.

How can be draw a line between animals that have just nociception and those that experience pain?

Clearly, many animals do not come even close to having a nervous system complex enough to produce the sensation of pain with its associated negative emotions. Animals like the pond snail (11,000 neurons) or the sea slug (28,000 neurons) just don’t have this capacity. By comparison, we have 100 million neurons just in our gut (the enteric nervous system) and 86 billion neurons in our brain.

A table of the number of neurons in different animal species can be found here.

Among the invertebrates, the only animal that has a fairly complex nervous system is the octopus, with 300 million neurons, comparable with the rat’s 200 million neurons. This is why countries like the UK and Canada now give cephalopods (octopi, squids and cuttlefish) the same protections given to vertebrates.

However, the number of neurons should not be the only metric to measure the complexity of a nervous system. Thus, the neurons of the octopus and other cephalopods do not have a myelin coating in their axons, so they send information much more slowly than the vertebrates. We need to use other metrics, like the number of synapses or overall capacity to process information.

But what most people are concerned about are the most complex animals - mammals and birds - which we eat, have as pets and use in scientific research.

What about them? Do they feel pain? Do they suffer?

The sensory and emotional aspects of pain

In mammals, a lot can be learned about the relationship between pain and suffering by studying brain areas involved in the processing of pain.

As I said above, pain has a sensory aspect and an emotional aspect.

The sensory aspect of pain is processed by the somatosensory cortex, an area shaped like a hairband going from the top to the sides of the brain. It contains a detailed map of the body and processes pain and touch, telling us where these sensations originate. Nowadays, it is recognized that the dorsal posterior insula also contains a map of the body and handles judgements on the localization and intensity of pain. The somatosensory cortex is connected to the orbitofrontal cortex, located at the front end of the brain and whose function is to plan actions according with the information it receives.

Sagittal section of the human brain highlighting the cingulate cortex.
The cingulate cortex in the human brain.

But neither the somatosensory nor the orbitofrontal cortex are responsible for the emotional component of pain. This function is assigned to two other areas of the cortex: the insula and the anterior cingulate cortex (ACC). Generally speaking, the function of the insula is to tell us how bad pain feels and to associate that emotion with a host of other emotions like sadness, fear, anger, joy, disgust and pleasure.

Emotions can be understood as motivational states of the brain. They predispose us to act in a certain way, organizing everything we feel in a hierarchical way according to what takes priority for action.

Pain is an emotion that motivates us to stop or escape from whatever is hurting us. This urgent motivational aspect of pain is processed by the ACC. Therefore, we could say that the insula and the ACC work together to turn pain into suffering by giving it its “I don’t like it” and “I want to stop it” qualities.

Pain processing is special in humans

Recent discoveries have revealed that during the evolution of primates (monkeys, apes and humans) there was a reconfiguration of the brain pathways that process pain, culminating with the appearance of completely new pain processing areas in the human brain (Craig, 2003, 2010).

Noxious signals are carried by specialized fibers in the nerves from any part of the body to the dorsal horn of the spinal cord. From there, the signals travel to the parabrachial nucleus in the brain stem, where they branch out to different nuclei of the thalamus and the forebrain (Craig, 2003).

Diagram of the neuronal pathways of pain.
Pain neuronal pathways. Craig, AD. Nature Reviews in Neuroscience 3:655-666, 2002

Located in the middle of the brain, the thalamus functions as the central relay of all sensory information. Its different parts, or nuclei, handle visual, auditory, gustatory, tactile and pain information. Different thalamic nuclei send pain signals to the four areas of the cortex mentioned above: the somatosensory cortex, the orbitofrontal cortex, the insula and the ACC.

These pain pathways are present in all mammals, but in primates a new additional pathway emerged that directly links the spinal cord with the nucleus of the thalamus connected to the insula, bypassing the parabrachial nucleus. This means that pain sensations are able to reach directly the part of the cortex where feelings are created.

In humans, the size of this direct pathway between the thalamus and the insula is much larger and more complex than in monkeys.

The anterior insula

There is another change in the brain unique to humans, which is not found even in monkeys. It’s a new region of the brain called the anterior insula (Craig, 2011; Bauernfeind et al., 2013).

A. D. Craig is a scientist who has studied these changes by mapping the brains of monkeys, apes and humans. He thinks that the posterior insula serves to create an emotional map of the state of the body at each moment. The anterior insula, in contrast, serves to model the state of the body in hypothetical situations: “if this were to happen, this is what I would feel.”

Craig proposes that the anterior insula mediates self-awareness by modeling feelings that represent the interior state of the body. The representation of hypothetical states of the body performed by the anterior insula is also responsible for empathy: the ability to feel what another person is feeling. The anterior insula does that by simulating their body state in our own brain. The gradual appearance of the anterior insula in apes like bonobos and chimpanzees correlates with the development of empathy and positive social emotions (Rilling et al., 2012; Bauernfeind et al., 2013).

Therefore, as the mammalian brain evolved into the human brain, the insula became more relevant in generating the negative emotions associated with pain. This increased the depth of suffering. Two other unique properties of the human mind, extended consciousness and theory of mind, contribute to this.

Extended consciousness means that not only we experience pain in the present moment, as animals do, but we are also aware of having suffered in the past and that we may suffer in the future. Animals that lack an anterior insula would not be able to experience suffering as something that extends into the past and the future.

Although animals have memories, without the anterior insula they cannot use them to construct a vivid representation of their past suffering, like we do. They do not have deep suffering. A measure of self-awareness and deep suffering may exists in elephants and cetaceans, which also have a developed anterior insula and ACC with von Economo neurons.

Conclusion: some animals suffer but only humans have deep suffering

We need to take a gradualist approach when considering the existence of pain and suffering in animals.

Invertebrates, with the possible exception of cephalopods, do not appear to have a nervous system complex enough to feel pain, let alone experiencing suffering. Their behavior can be explained by automatic responses to nociceptive signals.

Vertebrates, particularly the ones with highly complex nervous systems like mammals and birds, do experience pain and probably suffer from it. However, the deep suffering that we experience as humans beings, rooted in our extended consciousness and our capacity to imagine the future, does not seem to exist in other mammals.

Jeremy Bentham and Peter Singer failed to understand the true nature of suffering when they came up with the idea of speciesism. Just as we do not give the same moral status to animals and plants, we cannot give the same moral status to all animal species.

When deciding how we should treat animals, we need to take into consideration whether they can feel pain and, if they do, whether they suffer from that pain.

The suffering of a mouse, a dog, a monkey and a chimpanzee are not equivalent. By the same token, human suffering has to be given a higher ethical consideration than the suffering of other animals.

There is a moral imperative to diminish suffering in all sentient beings, but when difficult choices have to be made, human suffering has to come first.

If saying this makes me a speciecist, I will wear that label with pride. But I’d rather call myself a humanist, because for me the priority is to decrease human suffering.


  • Bauernfeind AL, de Sousa AA, Avasthi T, Dobson SD, Raghanti MA, Lewandowski AH, Zilles K, Semendeferi K, Allman JM, Craig A.D., Hof PR, Sherwood CC (2013) A volumetric comparison of the insular cortex and its subregions in primates. J Hum Evol 64:263-279.

  • Craig A.D. (2003) Interoception: the sense of the physiological condition of the body. Curr Opin Neurobiol 13:500-505.

  • Craig A.D. (2010) The sentient self. Brain Struct Funct 214:563-577.

  • Craig A.D. (2011) Significance of the insula for the evolution of human awareness of feelings from the body. Ann N Y Acad Sci 1225:72-82.

  • Craig A.D. (2014) Topographically organized projection to posterior insular cortex from the posterior portion of the ventral medial nucleus in the long-tailed macaque monkey. J Comp Neurol 522:36-63.

  • Rilling JK, Scholz J, Preuss TM, Glasser MF, Errangi BK, Behrens TE (2012) Differences between chimpanzees and bonobos in neural systems supporting social cognition. Soc Cogn Affect Neurosci 7:369-379.

Recent Posts

See All


bottom of page