Should animals have the same basic legal rights as people? Yes, says Steven Wise, who was incensed by scientists' treatment of a young chimp.

In the mid-1980s, a tiny group of Londoners accused the McDonald's Corporation and its British subsidiary, McDonald's Restaurants Limited, of cruelty to the animals whose bodies they sold for food. After a defamation trial that lasted 313 days, during which the two defendants represented themselves, the chief justice of the English High Court of Justice, Queen's Bench Division, found these corporate giants "culpably responsible" for much of the alleged cruelty.

The 1997 decision in what became known as the "McLibel" case covered 800 pages. Chief Justice Bell's detailed findings of cruel, yet common, corporate agricultural practices were made more chilling by the detached and legalistic tone of his litany of cramped conditions, broken and dislocated bones, crushed heads, inhumane gassings and slaughterhouse terror. He explained that a farming practice could be both cruel and legal and rejected the argument put forward by McDonald's that every customary farming practice should be deemed acceptable, for "to do so would be to hand the decision as to what is cruel to the food industry completely". Much cruelty, the judge found, "could be avoided if less attention was paid to profit and high production and more to the animals".

None of this was new to anyone who had read Animal Liberation by Australian philosopher Peter Singer. After encountering it in 1979, I learned that tens of billions of animals are annually killed for food. Many are raised in abhorrent conditions on factory farms. Hundreds of millions languish and die in biomedical research or testing facilities. Hundreds of millions more are hunted, trapped, skinned or dissected. I learned that, while humans are "legal persons" with a vast number of legal rights, animals are "legal things". Our personhood operates as legal armour that protects us, while the legal thinghood of animals allows us to do with them as we will.

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Within the year, I began to shift my law practice towards animal protection and, throughout the next decade, steadily litigated in the interests of animals. Since then, I have argued that chimpanzees and bonobos are entitled to basic legal rights. Take Jerom, for example. He was a chimpanzee whom US biomedical researchers injected with multiple strains of HIV viruses, beginning when he was just two years old. They were hoping to kill him. They imprisoned him, along with a dozen other chimpanzee youngsters, first in a small, windowless, cinder-block building, then in a large windowless grey concrete and steel box, 9ft by 11ft by 8.5ft. After a decade, Jerom died a terrible and lingering death. The eminent Harvard constitutional law professor Laurence H. Tribe observed earlier this year that "clearly Jerom was enslaved".

Because it was "odious", the great Lord Mansfield refused to allow the 18th-century common law to sanction human slavery. This was the common law for which the 19th-century chief justice of the Supreme Judicial Court of Massachusetts, Lemuel Shaw, provided a working definition. It consists, he said, "of a few broad and comprehensive principles, founded on reason, natural justice, and enlightened public policy, modified and adapted to all the circumstances of all the particular cases that fall within it". As the common law abhorred human slavery, I suspected that it could also function as an instrument to crush some of the worst of non-human slavery.

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I concluded that whether the astonishingly large number of animals whom we annually torment and kill should continue as "legal things" was one of the great moral and legal issues of the age. As the 19th century melted into the 20th century, hadn't the American judge Oliver Wendell Homes, Jr lectured on the law almost within shouting distance of my Boston apartment?: "It is revolting to have no better reason for a rule of law than that it was laid down in the time of Henry IV. It is still more revolting if the grounds upon which it was laid down have vanished long since, and the rule persists from blind imitation of the past." "Precisely," I thought.

Most judges, lawyers and law professors knew little of the story of how the legal thinghood of animals had emerged from ancient ways of understanding the universe. They had no idea that we know today that these ancient understandings were false.

Legal instruction that concerned animals was scattered among traditional law school classes in property, wills and trusts, criminal law, torts and contracts. Even environmental law courses failed to address the interests of individual animals, valuing them only as cogs in an ecosystem. With the exception of a course offered at the Pace University Law School in New York in the mid-1980s, "animal protection law" - law able to advance the interests of individual animals within a legal system that considers them to be legal things - did not formally exist in any university's law curriculum, though it was being practised.

"Animal rights law", the idea that at least some animals should possess at least some basic legal rights, was never discussed. No US law school course had systematically addressed what we do to animals, how it came about, whether what we do can be justified in modern law and fact, and whether animals should be legal persons or legal things.

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How were judges, lawyers and law professors to understand the problems of the legal thinghood of animals in any systematic way unless it became an object of formal study?

In 1990, I received a call from the Vermont Law School. Its environmental law programme was so respected that it could chance offering a summer course in such a novel field as animal rights law. Would I teach a course? I would. I called it "animal rights law". We would discuss which, if any, animals might be suitable to hold basic legal rights and what it was about them that made them suitable.

Over the next decade, tiny Vermont Law School proved an amiable incubator, as law students, even law professors and the occasional judge took this course. And not just from the US, but from Canada, South Africa and Mexico. Word got around. Some arrived passionately devoted to animal rights. Occasionally they were stoutly opposed. Often they were simply curious.

I realised that, as with most lawyers, judges and law professors, most law students knew little about how animals came to be legal things, knew almost nothing about what legal rights were, or moral rights, either. Without such understandings it would be senseless to argue either that animals should or should not have the basic legal rights, so we added to the study of history of how and why we use animals, meditations on what legal rights are, where they come from, why humans have them, which humans have them, what happens when they do not, why non-humans lack them, how and why humans who once lacked them attained them, and the suitability of the common law as a vehicle for attaining basic legal rights for at least some animals.

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Several years passed before a second US law school added a law and animals course. Then a third and a fourth were authorised, and a trickle became a stream. This year, nearly 20 courses are being offered at some of the best law schools in the US: Harvard, Yale, Georgetown, Northwestern, University of California at Los Angeles, Michigan and Hastings. I teach animal rights law at three law schools: Vermont, John Marshall (in Chicago) and Harvard. The Harvard's decision to offer the class has proved to be a watershed. First, it resulted from the sustained demand of Harvard law students over three years. Second, Harvard administrators have emphasised that the course is being offered not to make a political statement but to satisfy student interest in an increasingly complex and intellectually challenging field.

Today, law journals publish animal rights articles. When I appear in court, judges may ask if I am there on an animal rights case. Some even want to chat about animal rights law. But when invited to speak about legal careers at law schools, I am invariably placed on the environmental law panel. I want to sit with the civil rights advocates. I belong there because animal rights law draws from the same well of liberty and equality as does human rights.

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Steven M. Wise teaches animal rights law at Harvard, Vermont and John Marshall law schools. His Rattling the Cage: Toward Legal Rights for Animals, is published on May 22. To order a copy at the special price of Pounds 10.99 incl p&p (rrp Pounds 12.99), call Profile Direct on 020 8324 5530.