Study Guide: Peter Singer's 'Animal Liberation' All Animals are Equal

Introduction

Peter Singer’s influential 1975 book Animal Liberation—updated and republished in 2023 as Animal Liberation Now—is the philosophical cornerstone of the modern animal rights movement. The book’s foundational opening chapter, “All Animals Are Equal”, is widely assigned in philosophy courses as a classic expression of the view that “speciesism” is morally wrong for reasons parallel to racism, sexism, and other forms of unjust discrimination. It presents a powerful critique of our routine dismissal of the moral status of non-human animals that are (ab)used and exploited for our convenience and culinary pleasure. This study guide explains Singer’s argument in “All Animals Are Equal”, explores potential objections, and clarifies common misunderstandings. (For further discussion of this topic, see also our guest essay on Utilitarianism and Nonhuman Animals by Jeff Sebo.)

Singer’s Argument Against Speciesism

“Speciesism” refers to the practice of treating the interests of individuals of one species (typically humans) as inherently more important than those of others, without justification based on relevant differences in individual capacities or characteristics. The most common form of speciesism is the arbitrary privileging of humans over non-human animals, purely on the basis of species.

Singer analyzes what makes unjust discrimination in general wrong. He suggests that the common flaw in sexism, racism, and other objectionable discrimination is that it violates a principle of moral equality, specifically, the equal consideration of interests. An individual’s interests should not be discounted or disregarded based on unchosen group characteristics like their gender, race, or sexual orientation. Such factors don’t inherently affect an individual’s capacity for well-being, including how bad it is for them to suffer. Singer thus writes: “our concern for others and our readiness to consider their interests ought not to depend on what they are like or on what abilities they may possess.”¹

People are quick to rationalize their speciesism by appeal to some property—often intelligence or rationality—by which they take humans to be superior to non-human animals. The central problem with this move, as Singer points out, is that humans vary dramatically, including in their intelligence and rationality. Do smarter people inherently matter more, morally speaking? Would it be OK to do medical experiments on cognitively disabled children, or put them in factory farms to satisfy the culinary preferences of those with a taste for human meat? Surely not. But then the explanation of why not cannot depend upon their cognitive abilities (which they may possess to a lesser extent than many cognitively sophisticated non-human animals). This is known as the “argument from marginal cases” in philosophy, which has a long history—already in 1789, Jeremy Bentham famously asked:²

What else could be used to draw the line [between humans and non-human animals]? Is it the faculty of reason or the possession of language? But a full-grown horse or dog is incomparably more rational and conversable than an infant of a day, or a week, or even a month old. Even if that were not so, what difference would that make? The question is not Can they reason? Or Can they talk? but Can they suffer.”

Building on Bentham’s question, Singer agrees that:³

If a being suffers, there can be no moral justification for refusing to take that suffering into consideration. No matter what the nature of the being, the principle of equality requires that its suffering be counted equally with the like suffering—insofar as rough comparisons can be made—of any other being. If a being is not capable of suffering, or of experiencing pleasure or happiness, there is nothing to be counted. So the limit of sentience (using the term to indicate the capacity to experience pain or pleasure) is the only defensible boundary of concern for the interests of others.

Both Singer and Bentham thus argue that what’s really morally relevant when weighing the interests of different individuals is their capacity to suffer and experience happiness—which, as Singer notes, is necessary for having “interests” in the first place.⁴ Any other criterion, they argue, would be arbitrary and unjust. We all recognize when thinking about our fellow humans that it would be bad for them to suffer. But then, we should recognize that the same is true of non-humans who share our capacity for suffering, according to scientists and philosophers.⁵ We should not want them to suffer, either. And we especially should not do things that cause immense suffering, merely to obtain trivial (e.g. culinary) benefits for ourselves.

Factory Farming

Modern industrial meat production overwhelmingly occurs in factory farms (“concentrated animal feeding operations”), which impose huge suffering on more than 80 billion land-based animals (chickens, pig, cows, etc.) and up to 170 billion fish each year. These animals suffer immensely and often die prematurely from extreme confinement and crowding; chronic and infectious diseases; injury and cannibalism; physical mutilation (e.g. beak trimming in chickens and tail docking in pigs); health problems caused by selective breeding; lack of natural behaviors and social interaction; and inhumane slaughter practices. (For more information on the living conditions and number of factory-farmed animals, see this article from 80,000 Hours).

As such, factory farming is likely the largest source of human-caused suffering in the world and this suffering vastly outweighs the total benefits thereby produced. (And that’s before even considering the environmental and other costs.)⁶ Taking Singer’s arguments against speciesism seriously, the conclusion is clear: modern industrial meat production is morally indefensible and arguably among the greatest moral atrocities in history.

To make the comparison vivid, consider how much animal suffering goes into producing one serving of meat, on average. The details vary across different meat products. But the results are horrifying if you take animal welfare seriously. According to the researchers at Faunalytics, producing a single serving of chicken involves approximately six days of life in miserable conditions for a farmed chicken. Those conditions may involve “painful lesions and chemical burns” from “long periods of standing and lying in waste,” heat stress, and disease. If consumers had to endure that suffering themselves whenever they purchased meat, no-one would ever buy chicken nuggets again.

In general, you shouldn’t do something if you wouldn’t be willing to experience the resulting total harms and benefits yourself. If the only reason you find an outcome tolerable is because you get the benefits while unconsenting others suffer the costs, that’s a strong sign that you’re unjustly exploiting them. There are few clearer examples of this than factory-farmed meat.

Equal Consideration versus Identical Treatment

Perhaps the most common misunderstanding of Singer’s view is to mistakenly assume that, in calling for animal equality, he’s calling for animals to be treated identically to humans. The latter view is easy to dismiss. But in fact Singer is very careful to distinguish these concepts and recognizes that while species membership is not morally relevant in itself, members of different species may differ in other ways that do matter morally (e.g., in their capacity for conscious experience or their preferences). And these differences can justify differential treatment. Singer thus defends a more moderate view:⁷

The important differences between humans and other animals must give rise to some differences in the rights that each have. But there are also important differences between [human] adults and children. Since neither dogs nor young children can vote, neither has the right to vote. Recognizing this, however, does not count against extending a more basic principle of equality to children, or to nonhuman animals. That extension does not imply that we must treat everyone in exactly the same way, regardless of age or mental capacity. The basic principle of equality does not require equal or identical treatment; it requires equal consideration. Equal consideration for different beings may lead to different treatment and different rights.

What’s most important, for Singer’s purposes, is that humans and non-humans have a similar interest in avoiding suffering. So, by his principle of equal consideration, we have comparably strong reasons to prevent animal suffering as we do to prevent human suffering. And those are very strong reasons indeed.

This view is compatible with recognizing that people typically have myriad other interests that non-humans typically lack, including a much stronger interest in continued survival.⁸ Singer’s anti-speciesism is thus compatible with holding that if you could either save the life of a human child, or save the lives of a few chickens, you should save the child. This is because you might reasonably expect that the child will benefit more from life than the chickens (even added together).⁹ And it isn’t objectionably discriminatory to prioritize a greater benefit over a lesser one.¹⁰

Singer explains the application of his equal consideration principle to the ethics of killing as follows:¹¹

This does not mean that to avoid speciesism we must hold that it is as wrong to kill a dog as it is to kill a human being in full possession of their faculties. The only position that is irredeemably speciesist is the one that tries to make the boundary of the right to life run exactly parallel to the boundary of our own species. To avoid speciesism, we must allow that beings who are similar in all relevant respects have a similar right to life; and mere membership in our own species is not a morally relevant distinction on which to base this right. Within these limits we could still hold, for instance, that it is worse to kill an adult human with a capacity for self-awareness and the ability to plan for the future and have meaningful relations with others, than it is to kill a mouse, who presumably does not share all of these characteristics; or we might appeal to the close and long-lasting family and other personal ties that humans have but mice do not have to the same degree; or we might think that the crucial difference lies in the consequences for other humans, who will be put in fear for their own lives. Whatever criteria we choose, however, we will have to admit that they do not run parallel to the boundary of our own species. We may legitimately hold that there are some features that make it worse to kill most human beings than to kill a nonhuman animal, such as those just stated; but by any non-speciesist standard, many animals possess these features to a higher degree than…[some] humans with conditions that have profoundly and permanently impaired their cognitive abilities. Hence, if we base the right to life on these characteristics, we must grant these animals a right to life at least as strong as we grant to those humans.

Objections

Even once we take care to distinguish equal consideration from identical treatment, Singer’s view remains strikingly radical. It implies that, if a human is suffering intense pain, and a non-human animal is suffering even more intensely, and we only have enough painkillers on hand to help one of them, we morally ought to relieve whichever pain is worse—even if that means prioritizing a pig over a human.

This is a highly revisionary verdict, and many people find it counterintuitive. But merely not liking a moral verdict is not the same thing as having good reason to think that it is wrong. It might be that we find the verdict counterintuitive because we are objectionably speciesist, and Singer’s argument uncomfortably draws attention to this fact. Consider, for example, how sexists and racists also find principles like gender and racial equality counterintuitive and uncomfortable. Historically, moral progress has often involved revising some widespread and deeply held beliefs, even against the resistance of those benefiting from the unjust status quo.¹² If our present attitudes toward non-human animals conflict with compelling moral principles, it’s not obvious that the principles are to blame—maybe we should instead change our attitudes.

But some features of Singer’s view seem more open to principled criticism.

First, Singer assumes a strong form of moral individualism: that it does not matter what kind of being you are; all that matters are your individual characteristics. (Hence his view that species is morally irrelevant.) But one could question this assumption. To see this, suppose that you have a magic pill that will grant normal human intelligence to whoever takes it. And suppose you have two candidate recipients: a pig, and a cognitively disabled human whose cognitive abilities are comparable to the pig’s. Suppose (unrealistically) that “all else is equal”: either individual would similarly benefit from the extra intelligence, and nobody else will be affected at all. Should you bestow normal human intelligence upon the pig or the cognitively disabled human? Singer’s individualism implies that it makes no difference. But many people would have the sense that remedying a disability is more morally important than positively enhancing a perfectly healthy pig. Yet without appeal to a baseline of species-normality, there would seem no basis for distinguishing treatment from enhancement. Now, maybe that’s the right result, and our ordinary conceptual distinction here is actually baseless. But that result is quite radical, which might justify some skepticism that species membership is as morally irrelevant as Singer claims.

Second, we might distinguish discrimination that treats an individual worse than their objective moral status requires from discretion to treat an individual better than their objective moral status requires. The possibility of the latter casts doubt on Singer’s claim that “we must grant these animals a right to life at least as strong as we grant to [comparable] humans.” It may be that our society collectively grants rights to severely cognitively disabled humans that are more than is required by their objective moral status. It wouldn’t obviously follow that we were doing anything wrong to animals when we refrain from extending them the same privilege. So further argument may be required to fully defend Singer’s strong claims here.

But even if you are convinced by these objections, it’s important to keep them in perspective. Neither objection casts doubt on Singer’s argument that we currently (mis)treat animals in deeply immoral ways. In particular, neither objection constitutes a defense of factory farming. At most, they may suggest ways to disagree with Singer’s stronger claims about “speciesism” and demands for strictly equal consideration for animals. But his practical objections to the gross mistreatment of animals can go through even with much weaker theoretical underpinnings.

It may be especially worth emphasizing here that Singer’s objections to the mistreatment of animals do not depend upon accepting full-blown utilitarianism. It suffices to accept that the suffering of any innocent sentient being is seriously morally bad. And even Singer’s stronger claims about “speciesism” and equal consideration could easily be accepted by non-utilitarians who also accept deontic constraints, special obligations to friends and family (but not to strangers with whom one merely shares a demographic category), and prerogatives to do less than the best.

Practical Implications

Suppose you find Singer’s arguments against speciesism compelling. How should this impact your actions? Answering this question involves considering how you might most effectively reduce animal suffering (and also how this compares to other means of doing good, like protecting future generations).

To find the most effective ways to counteract speciesism and improve animal welfare, it’s useful to consider, first, the main causes of animal suffering, and second, the main levers to alleviate this suffering. Regarding the causes of animal suffering, Singer concludes the chapter by revisiting what he sees as the two most pressing examples of speciesism in practice:¹³

One of them—experimentation on animals—is promoted by our governments and often paid for by our taxes. The other—raising animals for food—is possible only because most people buy and eat its products. These practices are the heart of speciesism. They cause more suffering to more animals than anything else that human beings do. To stop them we must change what we eat, and change the policies of our governments as well. If these officially promoted forms of speciesism can be stopped, abolition of the other speciesist practices will not be far behind.

In chapter 4 of Animal Liberation Now, Singer endorses “effective altruism for animals”, doing whatever will help them most—including via one’s donations, career choice, and political actions. But he circles back to personal consumption choices as, on his view, ethically fundamental:¹⁴

All of the actions just mentioned are important things to do, but there is one more step we can take that underpins, makes consistent, and gives meaning to all our other activities on behalf of animals: We can take responsibility for our own lives, and make them as free of cruelty as we can. We can, as far as is reasonable and practical in our individual circumstances, stop buying and consuming meat and other animal products.

Changing our individual consumption (e.g., by going vegetarian or vegan) may be the most obvious practical response to Singer’s arguments. This doesn’t have to be all-or-nothing: as Singer acknowledges, reducing your consumption of animal products (proportionately) by 90% is 90% as good as going fully vegan, as far as the direct effects are concerned. So, if it’s easier to convince people to go “reducetarian” than fully vegan, that could make the more modest ask the better moral pitch—in much the same way that it can be better to ask people to donate 10% of their income to charity than 50%, even when the latter action would be morally better. Getting more people on board with incremental progress is often better than convincing a smaller number to act with greater moral purity.

While individual consumption choices are a salient target for moral theorizing, they are unlikely to be the most important lever available to us for reducing animal suffering. An even more promising option is donating to effective animal charities. It’s hard to precisely quantify the impact of such donations, and estimates vary wildly, but it’s quite plausible that a few hundred dollars a year could more than offset the harm of a standard meat-intensive diet.¹⁵ So if you would find it easier to donate (say) an extra thousand dollars to effective animal charities each year than to go vegan, that plausibly should take priority.¹⁶ (If you’re happy to do both, all the better!)

Conclusion

Animal Liberation, more than any other work of philosophy ever written, got philosophers and the general public thinking about animal ethics, the harms of factory farming, and the challenge of speciesism. At the heart of Singer’s challenge is a call for consistency: if we agree that racism, sexism, and other forms of unjust discrimination are wrong, and wrong because they violate the principle of equal consideration, how can we deny that the same is true of speciesism?

There are particular features of Singer’s view that could reasonably be questioned. But his core contention—that society’s current treatment of non-human animals, especially farmed animals, is morally indefensible—seems both undeniable and practically significant. If Singer is right, then society needs to change. And the only way that can happen is if we—the people who make up society—start to think and act differently as a result of appreciating these arguments.

Discussion Questions

• Would it be wrong to raise animals for food in genuinely humane conditions, where we could be confident that their lives are “worth living”, or positive on the whole, if the alternative is that these individual animals would not exist?
• Would it be wrong to raise extra human beings (who would otherwise not exist), in similar conditions, for the benefit of other people (whether cannibals or people in need of organs)? If not, why not? Does the same answer apply to non-human animals?
• Is there any principled reason to give more moral weight to cats, dogs, and horses than to pigs, chickens, and cows?
• If a human and non-human animal are suffering equal pain, should you flip a coin to decide who gets the last dose of pain relief?
• Are all human lives more worth saving than all non-human lives? Why or why not?
• Singer’s focus on individual consumption can sound surprisingly deontological for an avowed utilitarian. Do you think there is a tension between his focus on keeping our own hands clean (or “free of cruelty”) and his underlying utilitarianism? What is the most promising way to reconcile the two?

Resources and Further Reading

• Fischer, B. (2024). Weighing Animal Welfare: Comparing Well-Being Across Species. Oxford University Press.
• Kagan, S. (2019). How to Count Animals: More or Less. Oxford University Press.
• Kuruc, K. & McFadden, J. (2023). Monetizing the Externalities of Animal Agriculture: Insights from an Inclusive Welfare Function, Social Choice and Welfare.
• Nussbaum, M. (2024). Justice for Animals. Simon & Schuster.
• Regan, T. (2004). The Case for Animal Rights. Univ of California Press.
• Rowlands, M. (2025). Animal Rights. MIT Press.
• Sebo, J. (2023). Utilitarianism and Nonhuman Animals. In R.Y. Chappell, D. Meissner, and W. MacAskill (eds.), An Introduction to Utilitarianism.
• Setiya, Kieran (2018). Humanism. Journal of the American Philosophical Association 4 (4): 452-70.
• Singer P. (2023). Animal Liberation Now. Harper Collins.