A little over 30 years ago, an American lawyer named Steven Wise read a book that changed the course of his life.
Animal Liberation, by the Australian philosopher Peter Singer, argues that the perceived boundary between human beings and animals is arbitrary. It directed Wise down a path that he hopes will eventually lead to an animal being granted the same legal rights as a human being.
In 2007, Wise established the Nonhuman Rights Project, a charitable organisation that aims to prove that the way the legal systems of Western countries categorise animals is outdated and wrong. As it currently stands, they are viewed by the courts as legal "things" without the capacity to possess rights. Humans are legal "persons", so they can.
Wise and his supporters believe that this definition is fundamentally flawed, claiming that a growing mountain of scientific evidence proves that certain intelligent animals, such as chimpanzees, have similar cognitive abilities to humans and should be treated accordingly. During a recent visit to London, Wise told The Independent that the legal lot of "non-human animals" – a phrase he uses frequently to emphasise that humans are animals too – could be reasonably compared to that of America's black slaves.
"I began to work on animal-protection cases, but over the years I began to realise that all I could do was to help an animal here, an animal there, and that as a legal 'thing' they were essentially invisible to the civil law the same way that slaves were," he says. Last year, Wise decided that the time was right to lodge his first legal challenges. In December 2013, he filed writs of habeas corpus – used to seek freedom from unlawful detention – on behalf of four chimpanzees in the state of New York. "It took me 28 years before I was ready to file a suit and we thought the world was ready – both the legal world and the non-legal world," he said.
The cases have now progressed to the intermediate appellate courts in New York, with the next set of hearings due to begin in autumn. Wise hopes that next year they will reach the Court of Appeal. If he loses, he says, it doesn't matter – he views it as a war of attrition with the courts and is convinced that he will eventually win a landmark ruling which will send legal shockwaves around the world.
"We understand that judges know that this is something new, and that they may need to hear our arguments five times, 10 times, 20 times before they begin to accept them. But whether we win or whether we lose, we'll keep pushing forward because we're going to win."
He picked chimpanzees to be his test subjects because they have been studied by scientists for longer than the other intelligent species he has in his sights, so the body of evidence is impressive. For the chimps' cases, he persuaded nine scientists to file 100 pages of affidavits detailing more than 40 ways in which a chimpanzee's brain is exceptionally complex and similar to a human's.
As Wise puts it: "The non-human animals that we think we have the best chance of prevailing on are those that science can show are conscious, understand that they exist, think about themselves, can remember the past, plan for the future, and choose to live their lives in some of the ways that we can."
But the list doesn't stop with chimpanzees. Wise says there is currently enough scientific evidence to launch similar legal challenges on behalf of all the great apes, including orang-utans and gorillas, as well as both African and Indian elephants, orcas, whales and dolphins. The Nonhuman Rights Project has only three members of staff and about 60 volunteers, but its work has already raised enough eyebrows in the US to attract significant media attention, which Wise says has been crucial in attracting funding. Wise himself is due to appear on the current affairs show The Colbert Report next month. But the journey has not been an easy one.
"I used to have lawyers bark when I walked into a court room," he says. "But that doesn't happen any more. Over the years, we've found that people are increasingly sympathetic to what we're doing."
If Wise wins, it will be a historic moment – but he says that the implications will be "both wide and uncertain". Chimpanzees would no longer be able to be held in captivity or in zoos and would have to be moved to sanctuaries, he says, as they would probably not survive in the wild. In anticipation of this moment, he has already brokered agreements with some organisations who have agreed to take in any animals that are freed.
Ultimately though, he says the most important thing is to trigger a debate about which rights high-functioning animals should be granted. It is a discussion, he believes, "that's going to go on for ever, the way the debate about human rights goes on".
Zoos across the world will be watching the outcome of his four test cases with interest – and perhaps some concern.