YOUR story, Father of animal activism backs monkey testing (News, last week), has led some readers to conclude that I have changed my position on using animals in research.

Since I judge actions by their consequences, I have never said that no experiment on an animal can ever be justified. I do insist, however, that the interests of animals count among those consequences, and that we cannot justify giving less weight to the interests of nonhuman animals than we give to the similar interests of human beings.

If an experiment on a small number of animals can cure a disease that affects tens of thousands, it could be justifiable. Whether this is really the case in Professor Aziz’s experiments, about which I was asked in the BBC2 documentary Monkeys, Rats and Me: Animal Testing, is a question I have not studied sufficiently to offer an opinion about. Certainly it has been disputed. In my book Animal Liberation I propose asking experimenters who use animals if they would be prepared to carry out their experiments on human beings at a similar mental level — say, those born with irreversible brain damage.

I wonder if Professor Aziz would declare whether he considers such experiments justifiable. If he does not, perhaps he would explain why he thinks that benefits to a large number of human beings can outweigh harming animals, but cannot outweigh inflicting similar harm on humans.

A prejudice against taking the interests of beings seriously merely because they are not members of our species is no more defensible than similar prejudices based on race or sex.

Professor Peter Singer
University Center for Human Values
Princeton University