Peter Singer is a professional ethicist. Best known for his 1975 book "Animal Liberation" -- a canonical text of the animal rights movement and the inspiration for untold thousands to take up vegetarianism -- Singer, in the last quarter-plus century, has published a string of books on everything from test tube babies to the ethics of George W. Bush. Considered fearless by some, and dangerous by others, virtually all agree that he is among the most influential philosophers alive today.

Singer's ethics are strictly utilitarian. In his view, all actions are judged by the objective measure of suffering they cause; there's little place here for subjectivity. In his essay "Famine, Affluence, and Morality," for instance, he argues against the injustice of some people living in comfort while others starve. We have a moral obligation, he says, to do all we can to alleviate the suffering of others up to that point where the suffering of our sacrifice is equal to the suffering of those we are trying to help. (Singer himself donates 20 percent of his salary to Oxfam and UNICEF.) When confronted with the question of whether it's justifiable to save the life of one's daughter at the expense of the lives of two strangers, Singer's response is even more matter of fact. The choice, he would say, is a foregone conclusion: Two lives are better than one.

One expects such uncompromising arguments from college freshmen, but not the Ira W. DeCamp professor of bioethics at Princeton University. The difference is that in the course of 36 books, dozens of articles and countless lectures, Singer has thoughtfully backed up each of his arguments, and stuck to his guns for over 30 years. From a distance, his career seems a long, uphill, at times quixotic battle against humanity's latent selfishness. The emphasis on real-world application is the key to his appeal; his 1979 book, "Practical Ethics," widely regarded as a classic, reads like a handbook for how to live ethically in a morally complex age, taking on, in turn, abortion, capital punishment and income disparity, among many other common ethical conundrums. At the same time, Singer's unwavering focus on an ethical ideal often comes at the expense of real-world complexity, and can sometimes lend his arguments an air of absurdity, such as his remarks in 2001 in defense of bestiality, or his campaign to persuade the United Nations to award personhood to great apes.

Singer's new book, "The Way We Eat," co-written with Jim Mason, looks at the eating habits of three different American families: vegans, "conscientious omnivores" and a family eating the "standard American diet." The elements of each diet and the production chain that brought it to the table are then carefully considered in light of environmental impact, fair trade, the organic movement, the grow-local movement, genetically modified foods, animal rights and the depredations of agribusiness.

Salon recently caught up with Singer at the State of the Planet conference at Columbia University, where Singer was speaking on "Changing Values for a Sustainable World." Australian born, Singer is now 60 years old. His utilitarianism extends to his conversational style, which is measured and direct. He is not a hand-waver, and after enduring decades of attacks from one outraged group or another, he is not easily flappable. He is willing to entertain, for the sake of argument, virtually any suggestion. It's this very equanimity that can sometimes make conversation with him a bit maddening.

One of the things that distinguishes your new book is all the field research that went into it. What most shocked you, over the course of doing this research?

Probably this video I saw of this kosher slaughterhouse, AgriProcessors. I guess I had this idea that kosher slaughter is more strictly controlled than normal slaughter, and when you see that video and you see these cattle staggering around with their throats cut, and blood pouring out -- by no stretch of the imagination is this just a reflex movement. It goes on and on. And this happens repeatedly, with many different animals.

How are kosher animals supposed to be slaughtered?

They are supposed to be slaughtered with a single blow of a sharp knife across the throat. There's a virtually instant loss of consciousness, because the brain loses blood so quickly. That's the idea, anyway. But when you see this video, it's so far from that, I really did find it quite shocking.

You mention in your book that cows today produce three times as much milk as they did 50 years ago. That's a great advance, isn't it?

It is an advance, but you have to consider how this has been achieved. Fifty years ago, cows were basically fed on grass. They walked around and selected their food themselves, food that we can't eat, chewing it up and producing milk that we can eat. Now cows are confined indoors, and a lot of their food supply is grown specifically for them, on land that we could have used to grow food for ourselves. So it's actually less efficient, in that we could have gotten more food from the land if we didn't pass it through the cow.

Most of us have an idealized notion of what an organic farm is like. You visited an organic chicken farm in New Hampshire. Did it meet your expectations?

I have to say that it didn't. I guess I was expecting some access to pasture for the hens. When I got to this place, although it was in a beautiful green valley in New Hampshire, and it was a fine, sunny fall day, there were no hens outside at all. The hens were all in these huge sheds, about 20,000 hens in a single shed, and they were pretty crowded. The floor of the shed was basically a sea of brown hens, and when we asked about access to outdoors, we were shown a small dirt run which at the best of times I don't think the hens would be very interested in. In any case the doors were closed, and when we asked why, we were told that the producer was worried about bird flu. So, yes, it was not really what I expected. It was still a kind of a factory farm production -- although undoubtedly it was much better than a caged operation.

How much space are birds allotted in caged operations?

In the U.S., birds have as little as 48 square inches, a six- by eight-inch space. The United Egg Producers standards are gradually increasing over the next five years. We'll get up to 67 square inches. But that's still not the industry average, and even 67 square inches is just [the size of] a sheet of standard letter paper. In a cage, the birds are unable to stretch their wings. The wingspan of the bird is about 31 inches, so even if you lined one bird up on the diagonal, she wouldn't be able to spread her wings. And there's not just one bird in these cages, there are four or five. The weaker birds are unable to escape from the more aggressive birds. They end up rubbing against the wire and getting pecked, so they lose a lot of feathers, and they can't lay their eggs in the nesting box.

One good thing about this organic farm in New Hampshire is that there was this row of nesting boxes. It's been shown that hens have a strong instinct to lay in this kind of sheltered area. Conrad Lawrence, the science fiction writer and author of "The Council to Save the Planet," once compared requiring a hen to lay in an open space to asking a human to shit in public. They don't like it.

What if it were possible to genetically engineer a brainless bird, grown strictly for its meat? Do you feel that this would be ethically acceptable?

It would be an ethical improvement on the present system, because it would eliminate the suffering that these birds are feeling. That's the huge plus to me.

What if you could engineer a chicken with no wings, so less space would be required?

I guess that's an improvement too, assuming it doesn't have any residual instincts, like phantom pain. If you could eliminate various other chicken instincts, like its preference for laying eggs in a nest, that would be an improvement too.

It seems to come down to a trade-off between whether the bird has wing space or whether you can fit more birds in your shed, and therefore have to pay less heating costs. How does one go about weighing these alternatives? How does the ethicist put a price on the impulse of a chicken to spread its wings?

We recognize the chicken as another conscious being. It's different from us, but it has a life, and if something is really important for that chicken, if it would work hard to try to get it, and if we can give it without sacrificing something that's really important to us, then we should. If it's a big burden on us, that's surely different, but if it's a question of paying a few more cents for eggs, when we pay just as much if not more for a brand label we like, then we ought to be prepared to pay more for eggs so that the chicken can enjoy its life, and not be frustrated and deprived and miserable.

What constitutes a big burden? Doubtless the chicken farmer would say that building a larger shed or paying a bigger heating bill is a big burden.

It's only a burden to him if it harms his business, and it only harms his business if he can't sell the eggs he produces because other producers who don't follow those standards are selling eggs more cheaply. So, there's two ways around that: Either you have ethically motivated consumers who are prepared to pay a somewhat higher price for humanely certified eggs, or you cut out the unfair competition with regulations. Prohibiting cages, for example. And that's been done already, in Switzerland. And the entire European Union is already saying you can't keep hens as confined as American hens; it's on track to require nesting boxes, and areas to scratch, by 2012. So you can do it, and it doesn't mean that people can no longer afford to eat eggs.

In your book you discuss this in terms of the right of the chicken to express its natural behavior.

I tend not to put it in terms of rights, because philosophically I have doubts about the foundations of rights. But yes, I think these animals have natural behaviors, and generally speaking, their natural behaviors are the ones they have adapted for. And if we prevent them from performing those natural behaviors, we are likely to be frustrating them and making them miserable. So, yes, I think we ought to try to let them perform those natural behaviors.

Could you explain your position on "speciesism," and what this has to do with your call to "expand the circle"?

The argument, in essence, is that we have, over centuries of history, expanded the circle of beings whom we regard as morally significant. If you go back in time you'll find tribes that were essentially only concerned with their own tribal members. If you were a member of another tribe, you could be killed with impunity. When we got beyond that there were still boundaries to our moral sphere, but these were based on nationality, or race, or religious belief. Anyone outside those boundaries didn't count. Slavery is the best example here. If you were not a member of the European race, if you were African, specifically, you could be enslaved. So we got beyond that. We have expanded the circle beyond our own race and we reject as wrongful the idea that something like race or religion or gender can be a basis for claiming another being's interests count less than our own.

So the argument is that this is also an arbitrary stopping place; it's also a form of discrimination, which I call "speciesism," that has parallels with racism. I am not saying it's identical, but in both cases you have this group that has power over the outsiders, and develops an ideology that says, Those outside our circle don't matter, and therefore we can make use of them for our own convenience.

That is what we have done, and still do, with other species. They're effectively things; they're property that we can own, buy and sell. We use them as is convenient and we keep them in ways that suit us best, producing products we want at the cheapest prices. So my argument is simply that this is wrong, this is not justifiable if we want to defend the idea of human equality against those who have a narrower definition. I don't think we can say that somehow we, as humans, are the sole repository of all moral value, and that all beings beyond our species don't matter. I think they do matter, and we need to expand our moral consideration to take that into account.

So you are saying that expanding the circle to include other species is really no different than expanding it to include other races?

Yes, I think it's a constant progression, a broadening of that circle.

But surely there's a significant difference between a Jew, for instance, and a chicken. These are different orders of beings.

Well, of course, there's no argument about that. The question is whether saying that you are not a member of my kind, and that therefore I don't have to give consideration to your interests, is something that was said by the Nazis and the slave traders, and is also something that we are saying to other species. The question is, what is the relevant difference here? There is no doubt that there is a huge difference between human and nonhuman animals. But what we are overlooking is the fact that nonhuman animals are conscious beings, that they can suffer. And we ignore that suffering, just as the Nazis ignored the suffering of the Jews, or the slave traders ignored the suffering of the Africans. I'm not saying that it's the same sort of suffering. I am not saying that factory farming is the same as the Holocaust or the slave trade, but it's clear that there is an immense amount of suffering in it, and just as we think that the Nazis were wrong to ignore the suffering of their victims, so we are wrong to ignore the sufferings of our victims.

But how do you know at what point to stop expanding the circle?

I think it gets gray when you get beyond mammals, and certainly it gets grayer still when you get beyond vertebrates. That's something we don't know enough about yet. We don't understand the way the nervous systems of invertebrates work.

In your book you say that socially responsible folks in San Francisco would do better to buy their rice from Bangladesh than from local growers in California. Could you explain?

This is in reference to the local food movement, and the idea that you can save fossil fuels by not transporting food long distances. This is a widespread belief, and of course it has some basis. Other things being equal, if your food is grown locally, you will save on fossil fuels. But other things are often not equal. California rice is produced using artificial irrigation and fertilizer that involves energy use. Bangladeshi rice takes advantage of the natural flooding of the rivers and doesn't require artificial irrigation. It also doesn't involve as much synthetic fertilizer because the rivers wash down nutrients, so it's significantly less energy intensive to produce. Now, it's then shipped across the world, but shipping is an extremely fuel-efficient form of transport. You can ship something 10,000 miles for the same amount of fuel necessary to truck it 1,000 miles. So if you're getting your rice shipped to San Francisco from Bangladesh, fewer fossil fuels were used to get it there than if you bought it in California.

In the same vein, you argue that in the interests of alleviating world poverty, it's better to buy food from Kenya than to buy locally, even if the Kenyan farmer only gets 2 cents on the dollar.

My argument is that we should not necessarily buy locally, because if we do, we cut out the opportunity for the poorest countries to trade with us, and agriculture is one of the things they can do, and which can help them develop. The objection to this, which I quote from Brian Halweil, one of the leading advocates of the local movement, is that very little of the money actually gets back to the Kenyan farmer. But my calculations show that even if as little as 2 cents on the dollar gets back to the Kenyan farmer, that could make a bigger difference to the Kenyan grower than an entire dollar would to a local grower. It's the law of diminishing marginal utility. If you are only earning $300, 2 cents can make a bigger difference to you than a dollar can make to the person earning $30,000.

I wanted to list a few factoids that jumped out at me while reading your book, and if you want to comment on them I'd love to hear your thoughts. First, each of the 36 million cattle produced in the United States has eaten 66 pounds of chicken litter?

The chicken industry produces a vast amount of litter that the chickens are living on, which of course gets filled with the chicken excrement, and is cleaned maybe once a year. And then the question is, what you do with it? Well, it's been discovered that cattle will eat it. But the chickens get some slaughterhouse remnants in their feed, and some of that feed they may not eat, so the slaughterhouse remnants may also be in the chicken litter. So that could be a route by which mad-cow disease gets from these prohibited slaughterhouse products into the cattle, through this circuitous route.

Second factoid: 284 gallons of oil go into fattening a 1,250-pound cow for slaughter?

That's a figure from David Pimentel, a Cornell ecologist. The fossil fuel goes into the fertilizer used to fertilize these acres of grain, which are then harvested and processed and transported to the cattle for feed. We get back, at most, 10 percent of the food value of the grain that we put into the cattle. So we are just skimming this concentrated product off the top of a mountain of grain into which all this fossil fuel has gone.

So even if we all started driving Priuses we'd still have these cows to worry about.

Yes. In fact, there's a University of Chicago study that shows that if you switch from driving an American car to driving a Prius, you'll cut your carbon-dioxide emissions by one ton per year. But if you switch from a typical U.S. diet, about 28 percent of which comes from animal sources, to a vegan diet with the same number of calories, you'll cut your carbon-dioxide emissions by nearly 1.5 tons per year.

Third factoid: We have more people in prison in the United States than people whose primary occupation is working on a farm?

Isn't that amazing? Just as an example, when I wrote "Animal Liberation" 30 years ago or so, there were more than 600,000 independent pig farms in the U.S. Now there are only about 60,000. We're still producing just as many pigs, in fact more pigs, but there has been such concentration that we are now producing more pigs with a tenth as many pig farms. The same has happened in dairy and many other areas.

And finally, it turns out that a wood chipper is not the best way to dispose of 10,000 spent hens?

Yes, this also came to mind when you asked me what most shocked me. This was in San Diego County, in California. Neighbors noticed that a local chicken farm was getting rid of hens at the end of their laying period by throwing them by the bucketload down a wood chipper. They complained to the Animal Welfare Department, which investigated, and the chicken farmer told them that this was a recommendation that had been made by their vet, a vet who happens to sit on the Animal Welfare Committee of the American Veterinary Medical Association. The American Veterinary Medical Association, I should say, does not condone throwing hens down a wood chipper, but it is apparently done. We've also had examples of hens being taken off the conveyor belt and simply dumped into a bin, where by piling more hens on top, the hens on the bottom were suffocated. These old hens have no value, that's the problem, and so people have been killing them by whatever means is cheapest and most convenient.

So if you were stuck with 10,000 spent hens, what would you do with them?

I think you have a responsibility. Those hens have been producing eggs for you for a year or 18 months. You have a responsibility to make sure they are killed humanely. And you can do that. You can truck them to a place where there is stunning, or, better still, you can bring stunning equipment to the farm, and you can make sure that every hen is individually stunned with an electric shock and then killed by having its throat cut.

I thought you might suggest a retirement program.

That's an ideal that some people would like to see, but if you have to maintain and feed hens when they are no longer laying eggs, that will significantly increase the cost of the egg, and even the organic farms don't do that.

After reading this interview, some readers might be inspired to change their diets. If you could suggest one thing, what would it be?

Avoid factory farm products. The worst of all the things we talk about in the book is intensive animal agriculture. If you can be vegetarian or vegan that's ideal. If you can buy organic and vegan that's better still, and organic and fair trade and vegan, better still, but if that gets too difficult or too complicated, just ask yourself, Does this product come from intensive animal agriculture? If it does, avoid it, and then you will have achieved 80 percent of the good that you would have achieved if you followed every suggestion in the book.