If someone were to set up a sculpture park illustrating philosophers' thought experiments, a big part of the area devoted to modern ethics would have to be set aside for a shallow pond. In the pond there would be a statue of a floundering toddler - the central image of an argument that Peter Singer, an Australian philosopher and campaigner, has been making since the start of his career. Singer is best known for his views on our moral obligations to animals; his book Animal Liberation, published in 1975, is probably the single most influential document in the history of recent movements concerned with animal welfare. He has also written widely on medical ethics, and his positions on infanticide and euthanasia - both of which he considers justified under some circumstances - have been the object of campus protests in the US and Germany. But his earliest and perhaps most troubling argument has to do with extreme poverty in the developing world, a topic he returns to in his new book, The Life You Can Save.

Singer's argument, as first laid out in an essay in 1971, isn't hard to follow. "If it is in our power to prevent something very bad from happening, without thereby sacrificing anything morally significant, we ought, morally, to do it ... If I am walking past a shallow pond and see a child drowning in it, I ought to wade in and pull the child out. This will mean getting my clothes muddy, but this is insignificant, while the death of the child would presumably be a very bad thing." As he added, however, the "uncontroversial appearance" of this argument is deceptive. Considerations of distance, or of how many potential rescuers there might be, are irrelevant to Singer: the child you see dying of malnutrition or a preventable disease on the foreign news has as much of a claim on you as the child in the pond. Spending your surplus income on consumer treats rather than efforts to end extreme poverty, he concludes, isn't greatly different morally from leaving the toddler to drown.

People in rich countries who wish to act decently should, this implies, be giving away a much larger share of their earnings than almost any are willing to contemplate. (Singer himself gives around a quarter of his income to Oxfam and related groups.) Needless to say, this is a challenging position - "almost impossible to argue with", as the political theorist David Runciman once wrote, "but also very difficult to accept." Philosophers have expended considerable intellectual firepower on both Singer's specific arguments and the theories underpinning them, while practical objections have been raised to his early assumptions about the efficacy of giving. Even so, his case on poverty is still the one to beat if you're trying to defend the status quo, and a useful campaigning tool inside and outside academia. "I teach it to my undergraduates every year," Elinor Mason, a philosophy lecturer at Edinburgh University, says. "When I ask them at the end who has had their mind changed, at least 60% put their hands up. That's not a bad proportion."

Singer does not come across as a frivolous spender as he deals with offers of hot drinks in a meeting room at his British publisher's offices - "anything without caffeine in it would be great, but if you don't have that, just dip a teabag in and out". His black jacket and shirt look appropriate but inexpensive; his glasses don't sit quite straight on his nose; his raincoat and backpack might be characterised as utilitarian. When he laughs, the wrinkles around his eyes, combined with the effect of his Australian accent, give him a faint but disconcerting resemblance to Rupert Murdoch. This resemblance disappears when he's speaking, however. In spite of having just finished a publicity tour of the US - he has a part-time professorship at Princeton - he manages to seem practised rather than canned, somehow giving the impression that even his settled positions are always up for debate or yet another thinking through.

He's returning to his argument at book length now, he says, "because I really think that we have the capacity to make a big impact on extreme poverty in a way that we didn't in the 70s". He also wants to address repeated criticisms, and hopes he has become "more knowledgeable about some of the practical issues". (The book's detailed case for the improving effectiveness of aid programmes bears this out.) But his "main refinement is responding to one of the most common objections, which is that this is too demanding a morality, a morality for saints, not real, flesh-and-blood human beings. I've tried to listen to that. I think there's a sense in which the very demanding morality is still right, but it's also true that that is not going to be the basis of a moral code that you could get a large number of people to go along with."

As a result, he's proposing a scale of giving modelled on a progressive tax system. British people earning up to £68,000 a year, for example, are asked to pledge between 1% and 5% of their incomes to NGOs of their choice, with the percentages rising for earnings beyond various thresholds. Could his timing have been better? "Sure, but it's double-edged. I would rather have published at a time when people felt secure in their financial prospects. On the other hand, I think some people are starting to reassess their values, and if people do think about what's really important, maybe some of them won't go back to their consumption habits." Even more than raising money, "what I would like is to change the culture of giving, so that it becomes unacceptable to be comfortably off and do nothing for the world's poor. The sort of parallel here is with [his 1975 book] Animal Liberation. I had big hopes for it, and in the short term they were disappointed, but gradually, over the last 30-odd years, more things have started to happen."

Protecting your flanks in academia while addressing a wider public is notoriously difficult, but Singer has been unusually successful in combining academic clout with his campaigning. Jonathan Rée, a philosopher who first crossed paths with Singer in the early 70s, remembers him "making quite an impression by speaking up in defence of moralising. It seemed a brave and progressive thing to do at a time when philosophers were very wary about sticking their fingers into other people's pies." A lot of philosophers, he adds, "say mean things about him - he's too Australian, too brash, too successful - but the chances are they're just jealous. No one can take away the fact that, especially in the matter of the treatment of animals, he really has persuaded ordinary people to think differently."

Born in Melbourne in 1946 into a middle-class Jewish family, Singer had a more or less secular upbringing. Religious beliefs had no appeal; in his teens he wondered briefly if there was something watching over the universe, "but, ah, I concluded that there wasn't. And to make that conclusion, I wasn't reacting against anything that my parents strongly believed." As an undergraduate in Melbourne, he was involved with anti-Vietnam war movements, the Abortion Law Reform Association, and the left wing of the Australian Labor party. But his focus on animals and the global poor had its origins in his time as a graduate student in Oxford, where a fellow student, Richard Keshen, converted Singer and his wife Renata to vegetarianism and introduced them to some pioneering animal liberationists. He first wrote about poverty in response to the refugee crisis caused by the war that created Bangladesh.

After spending two years as a lecturer in Oxford, Singer wrote the bulk of Animal Liberation as an assistant professor at New York University, then returned to Australia. In 1999, after 22 years as a professor at Monash University in Melbourne, he accepted a position at Princeton's Center for Human Values. (The British philosopher Bernard Williams, a long-term critic of Singer's concept of "speciesism", joked in an essay: "I should have thought it would have sounded to him rather like a Center for Aryan Values.") He also stood unsuccessfully as a Green party candidate for the Australian senate in 1996, though "you have to realise that we have a preferential voting system, so it's not as if by standing for the Greens you help the conservatives to get elected." These days he divides his time between Princeton and Melbourne, "an arrangement I've worked out with my wife that makes it possible for us to stay close to our children and grandchildren." Does he offset the flights? "I do, generally. Obviously it would be better not to fly so much."

There's a feeling among philosophers that, especially in Oxford, Singer - as Rée puts it - "led a certain revolt against the perceived sterility of the Anglo-American philosophy of the 60s". Singer goes along with this, up to a point: "There was already a little bit of change in the air, and certainly people like RM Hare, my teacher, were quite open to me tackling more applied topics. So I was pushing on a door that, if not open, was certainly not locked." In those days, he was briefly one of the young, leftwing thinkers associated with the journal Radical Philosophy. "I thought that was great at first, but I was arguing that the way in which philosophy should be radical was to get involved with practical issues and write about them in a way that reaches a lot of people. For a lot of people in the group, though, it seemed that the way to be radical was to talk about Heidegger or Althusserian Marxism, and I thought that was just taking it in an academic direction."

He can also be as dismissive as any 60s analytical philosopher when it comes to the "continental" school of thinking. In January, he and Paola Cavalieri published a majestically disdainful reply to the Marxist-Lacanian writer Slavojek, who had called Singer, among other things, one of the great philosophers of today's global capitalism. "What we did, I think," Singer says carefully, "was to expose some fallacies and lack of clear thinking in his piece. I haven't had a response from him. I think that continental philosophy, in this area of animals, has really failed to live up to its own claims to be the more radical approach. If you look at what all those philosophers say, they end up being much more conservative and accepting of conventional views about animals than a substantial body of anglophone philosophy, which has been more critical and has actually inspired political movements to change the status quo."

Singer's own approach to ethics, a version of utilitarianism, has deep roots in the English-language tradition, but it's scarcely uncontroversial. One famous criticism, associated with Williams, is that it's implausibly demanding, making people as responsible for the things they fail to do as the things they bring about. Williams's ultimate point was highly technical; Singer, in discussing it, soon brings the argument back to practical outcomes. "I think we can set standards that limit our responsibilities to help people. But I wouldn't want to say, therefore we're only responsible for our acts and not for our omissions. If you draw a hard line there, you end up saying that really quite trivial things are wrong because they're violations of my positive responsibility not to cheat or whatever ..." He casts about for an example. "Well, we have it all over the tabloids, don't we: I charged the government £5 for watching porn movies, right? I had the opportunity to save a child's life, either by ruining my shoes in the pond or by giving some spare money I had to Oxfam, but somehow that's not as important to assessing whether I'm a decent person or not as whether I cheated the government out of £5 to watch a porn movie. And I think that's the wrong set of priorities, that sends the wrong sort of message."

Another criticism, often made in the 19th century, is that utilitarianism is aggressively philistine. I mention Dostoevsky having a character he despised argue that boots are better than Pushkin on the grounds that "one can do very well without Pushkin", but not without boots. "But that's a very crude caricature of utilitarianism," Singer says. "If reading Pushkin gives you more pleasure than walking in good boots, then you should read Pushkin." (Walking holidays are his only remotely guilty pleasure.) What if you're some kind of bootless serf? "Well, that will depend on what will most benefit the masses, won't it? I mean, ideally they'll all have some sort of footwear, and then you can ... introduce them to the pleasures of Pushkin as well."

Singer is troubled by people's tendency to care more about their children, families and neighbours than about distant strangers. Even when writing Pushing Time Away, his 1999 book about his grandfather, a friend of Freud and Adler who was killed in the Holocaust, he worried that there were "more important issues around. I suppose I just accepted that these family ties were of interest to me - and," he adds with extreme reluctance, "you could say that that has some connection with the discussion in the new book about obligations to your own kin rather than to ... impartiality. But only on the level of recognising the psychological complexity of our makeup - not in the sense of thinking that this gives you a moral justification for giving substantially greater weight to the interests of those close to you than to those further away."

The world, he writes in The Life You Can Save, "would be a much simpler place if one could bring about social change merely by making a logically consistent moral argument." Have his views on the power of argument changed over the years? "Yeah, you could say that I've become more of a realist about that. When I published Animal Liberation, I thought - and I still think - that the argument was completely irrefutable, rationally, and that people should have just said, 'Oh, yes, well, this is obviously true, we've got to become vegetarian or vegan and change many things.' Well, some people have done that - I have no idea what the tally is, but it must be tens or hundreds of thousands of people. But, you know, it's still a minority view."

What scale of response to the new book would satisfy him? Singer laughs through his nose. "Look, I don't really know in terms of figures." If the number of pledges on his website reach five figures, "that would please me, but I don't know about completely satisfy me. That would" - his voice rises in a way that's humorous but also wistful - "have to be many millions, I think, to completely satisfy me."

Singer on Singer

"Really, Dad, sometimes you let philosophy carry you away. Too much reasoning, not enough feeling. That's a horrible thought." ..."You know very well that I care about Max, so lay off with the 'You reason, you don't feel' stuff, please. I feel, but I also think what I feel. When people say we should only feel - and at times Costello comes close to that in her lecture - I'm reminded of Göring, who said, 'I think with my blood.' See where it led him. We can't take our feelings as moral data, immune from rational criticism."

This is one of the very few pieces of fiction that I've ever written. Coetzee's lecture was a difficult piece to respond to, because I didn't know whether I should respond to the character of Elizabeth Costello (who is obviously like Coetzee in some ways: someone coming from a southern hemisphere, former British colony to an elite American university to give a series of lectures) assuming that she's speaking for Coetzee, or not. I didn't know quite how to take it, and I thought writing my response as a piece of fiction in a way does the same thing and eliminates those problems, because then whatever can be said about whether Costello represents Coetzee could be said about whether my character represents me.