Of all the questions about who counts, the moral status of animals touches daily life most directly: what we eat, what we wear, how medicines are tested and how wildlife is managed.

Older views

The French philosopher René Descartes (1596 to 1650) is often read as treating animals as complex machines without feelings, though scholars debate how far he went. Immanuel Kant (1724 to 1804) held that we have no duties directly to animals, but that cruelty to animals is wrong because it hardens people and makes them more likely to be cruel to humans. This is called an indirect duty view.

Bentham: can they suffer?

The English philosopher Jeremy Bentham (1748 to 1832), a founder of utilitarianism, took a different view. In a footnote to his Introduction to the Principles of Morals and Legislation (1789), he compared the treatment of animals to slavery and wrote: “The question is not, Can they reason? nor, Can they talk? but, Can they suffer?” For Bentham, the capacity to feel pain and pleasure, called sentience, is what makes a being's interests count.

Singer: animal liberation

The Australian philosopher Peter Singer developed Bentham's idea in Animal Liberation (1975), one of the most influential books in modern ethics. His central principle is the equal consideration of interests: similar interests deserve similar weight, whatever species the being belongs to. A pig's pain counts as much as a comparable pain in a human. This does not mean treating animals and humans the same; a pig has no interest in voting. But it rules out discounting suffering simply because of species.

Singer popularized the term speciesism (coined by the British psychologist Richard Ryder in 1970) for giving less weight to a being's interests merely because it belongs to another species, which he compared to racism and sexism. He argued that industrial animal farming and much animal experimentation cause enormous suffering for relatively small human benefits. Because Singer is a utilitarian, his view weighs outcomes: an experiment that caused some suffering might be justified if it prevented much greater suffering.

Regan: animal rights

The American philosopher Tom Regan rejected the utilitarian approach in The Case for Animal Rights (1983). He argued that many animals, at least mentally normal mammals a year or more old, are subjects-of-a-life: beings with beliefs, desires, memory, a sense of their own future and a welfare that matters to them. Such beings have inherent value and rights, so they must never be treated merely as means, even to produce good outcomes. Regan therefore called for abolishing, not just reforming, commercial animal agriculture, commercial hunting and animal experimentation. The key contrast: Singer weighs outcomes; Regan holds that rights cannot be overridden simply because the benefits are large.

Critics and alternatives

• Some philosophers, such as Carl Cohen, argued that rights belong to members of a moral community who can understand and make moral claims, which animals cannot. Defenders of animals reply that infants and some disabled humans cannot do this either, yet clearly have rights.
• Many people defend a welfare position: using animals can be acceptable, but causing unnecessary suffering is wrong. Laws against cruelty and the research principle of the three Rs (replace, reduce and refine animal use, proposed by William Russell and Rex Burch in 1959) reflect this middle view.
• Environmental critics point out that animal ethics focuses on individual sentient animals, while ecology cares about species and ecosystems. Removing invasive rats to save seabirds may be good for the ecosystem yet harms individual animals. This tension leads to the next level.

Worked example: rats on a seabird island

Imagine a remote island where seabirds have nested for thousands of years. Ships accidentally brought rats, which now eat the eggs and chicks. Two rare bird species are close to disappearing. A conservation group proposes poisoning every rat on the island. How would the views in this level judge the plan?

A welfare view would accept that action is needed but insist on reducing suffering: choose the most humane method available and avoid harming other animals along the way.

Singer's utilitarian approach counts everyone's suffering equally. The rats will suffer and die; so will huge numbers of chicks, year after year, if nothing is done. If the poisoning prevents more suffering than it causes, and no gentler option such as trapping would work, the plan may be justified. But the rats' pain cannot be ignored just because they are rats, and the birds' rarity does not by itself make their suffering count for more.

Regan's rights view makes the plan much harder to defend. If the rats are subjects-of-a-life, killing them to benefit others treats them merely as means. On his view, the birds' rarity does not give anyone the right to kill the rats.

An ecosystem perspective (Level 4) starts elsewhere: the rats do not belong to this ecosystem, and restoring its health may matter more than any individual animal.

Conservation agencies have carried out many such removals, and many seabird colonies have recovered. The case shows why animal ethicists and environmentalists sometimes find themselves on opposite sides.