Two major philosophers prior to Bentham — René Descartes and Immanuel Kant — had little regard for the rights or even well-being of animals. But Bentham thought that pleasure and pain were primary considerations in the well-being of all sentient beings (beings possessing sensations), not only humans.

Early Indifference to Animals

It is hard to imagine a philosopher being more indifferent to the well-being of animals than René Descartes. According to Descartes, human beings possess the distinguishing feature of being res cogitans, or “thinking stuff.” That is, humans possess a substantial mind or soul. This mind is the locus of feelings, thought, rationality, and is the basis for free will and moral values. By contrast, Descartes thought that animals have no mind or soul and are therefore ultimately only res extensa, or “extended, physical, stuff.” Animals are essentially just fleshy machines or automata merely. No “soul” was reflected in their eyes and similarly no real pain was reflected in their apparent pain behavior.

As a consequence of lacking souls they also lack consciousness and cannot feel pleasure or pain. Descartes's teaching is also Christian doctrine, and Descartes accepted the teaching that humans have souls created by God but animals do not. And since soul is identical to mind, if animals have no soul, they are also lacking in mind. And if they have no mind, they cannot feel pain. In order to feel pain a mind is needed.

After Descartes, there were mixed views about the rights and well-being of animals during the Enlightenment. In his Lectures on Ethics Kant told his students:

So far as animals are concerned, we have no direct duties. Animals are not self-conscious, and are there merely as a means to an end. Their end is man.

Here Kant adopts the same view that Descartes does in asserting that animals are lacking in consciousness. Even the second version of Kant's categorical imperative enjoins people to “treat other persons (or rational beings) as ends but never as means.” But Kant allows humans to treat non-humans as they will.

The First Champion of Animal Rights

In 1780, the same year that Kant's Lectures were published, Bentham completed his Introduction to the Principles of Morals and Legislation. Speaking of animals, Bentham answered Kant definitively:

The question is not, Can they reason? nor, Can they talk? but, Can they suffer?

Here Bentham the utilitarian shows that rationality is secondary to well-being and value. Pleasures and pains form the foundation of a person's treatment toward sentient beings. Bentham implies that if you cause pain to sentient beings, then you have acted immorally. So in this way utilitarianism comes across as a more compassionate philosophy than Kant's deontology.

In the Old Testament it is stated that man shall have “dominion” over the animals. Bentham may have been the first to denounce that dominion and call it tyranny.

Animal Rights: Bentham's Legacy

Some animal rights detractors claim that nonhumans cannot possess rights. Even if this is true, it doesn't erase your duty not to harm them. You may have a duty, for example, not to destroy a famous building or religious statue, but not because the items have rights to exist. Few philosophers, if any, have carried on the spirit of Bentham's defense of animals more than Peter Singer. A philosophy professor at Princeton, Singer has made a strong case for the view that animals' interests are the basis for their having rights — rights that are equal to humans. According to Singer, not to respect the interests of animals is “speciesism.” He believes this is an objectionable attitude similar to racism or sexism — objectionable because it treats animals badly simply because they are members of a different species and gives preference to our own species just because we are human beings. But on what grounds is this objectionable?

According to Singer, having interests is connected to the ability to feel pleasure and pain, because the pleasure is derived from the satisfaction of an interest. Animals are different from plants in this regard. Plants have things that are in their interest even though they do not have interests. Because the interests of animals are similar to ours, they ought to be given equal weight according to Singer. On the one hand, it would make no sense to say that a pig or a horse has a right to vote, because it has no interest in voting. However, according to Singer, it would make sense to say that they have a right not to suffer or not to suffer needlessly or not to be used for no good purpose.

Others argue that animals need not be treated as equal to humans and that their interests ought not to be given equal weight with ours. It is because of the difference in species' abilities and potentialities that animals are a lesser form of being, according to this view. This does not mean, however, that interests ought to be disregarded.

Whether or not something is cruel to an animal might be determined by the extent of the pain it experiences, as when you speak of cruelty in terms of “unnecessary” pain. But Singer found that many experiments that made use of animals as guinea pigs to test products ranging from vaccines to shampoos to antifreeze were cruel and immoral. No doubt Bentham would have drawn the same conclusion.