The Maitreya Social restaurant in Bristol would seem to be the ideal location to talk to Peter Singer, the author of Animal Liberation and one of the world’s most celebrated moral philosophers. Vegetarian and organ­ic, it is impeccably ethical. You can even ­settle your bill with Bristol Pounds, the local currency, designed to keep wealth out of the hands of multinational businesses and big banks. But according to my guest’s austere utilitarian principles, there is no justification for spending £10.95 on a summer vegetable galette when people on the other side of the world are dying of malnutrition.

I’m meeting Singer because, along with Thomas Pogge, he is one of the leading intellectual inspirations of effective altruism, the central principle of which is summed up in the title of his latest book, The Most Good You Can Do (Yale University Press). The idea is simple: that when you donate to charity, you ought only to give to organisations which can demonstrate that they will make good use of it. This has suddenly become a very hot topic, with the concern in recent weeks that donors to Kids Company were moved more by the charisma and dedication of its founder rather than hard evidence that it was spending money well.

The time seems right for effective altruism. Sociologists such as the late Ulrich Beck have charted the rise of post-materialist values in developed countries for years. The idea that money and possessions do not make us happy has become the new common sense. Instead, people now seek experiences as well as a kind of spirituality-lite, such as the secular practice of mindfulness. No wonder the scientist and Buddhist monk Matthieu Ricard’s book Altruism has become a huge bestseller. The promise of its subtitle is that, by developing compassion, you can “change yourself and the world”.

At the same time, with refugees at Europe’s door and the iniquities of international trade more widely known than ever, we are acutely aware that our ennui with shiny new things is obscene compared to the misery of the world’s poor. We have also seen how decades of aid have borne surprisingly little fruit. Effective altruism speaks both to our sense of obligation to help and to our frustration at being unable to do so.

Effective altruists are more challenging when they stress the quantity as well as the quality of our giving. The two founders of the movement, the young Oxford philosophers Toby Ord and Will MacAskill, set up Giving What We Can (givingwhatwecan.org) in 2009 to recruit members who would pledge to give at least 10 per cent of their income to effective good causes. To date, 1,138 have done so. This might seem fairly demanding, but for many people 10 per cent is just a pragmatic compromise. As Singer puts it, “In order to get movement in the right direction we need to praise people who are doing significantly more than most of their peers are doing.” Yet leading figures in the movement believe most people should give a lot more. Ord has given away over a third of what he has earned and is on track to donate £1m over his lifetime, while MacAskill expects to give away 60 per cent of his lifetime earnings.

The emphasis on maximising the amount you give also has a surprising corollary. If you enter a caring profession, you make the world better only by the margin of how much better you do that job compared to the other person who would otherwise have taken it. If you become rich on Wall Street and give a lot away, however, the difference you make is huge, because the candidate you beat would probably have spent it on fast cars and in overpriced restaurants.

For those of us who want neither to work in the City nor to give up the odd plate of chilli and sun-blushed tomato farinata, it is tempting to ignore the call to give more and to focus solely on the call to give better. But according to the utilitarian philosophy that draws the likes of Singer and Ord to effective altruism, these things are two sides of the same coin.

Start with the principle that charity should work. In one sense this is unarguable and long overdue. For instance, in his book Doing Good Better (Guardian Faber), MacAskill tells the cautionary tale of the PlayPump, a water pump driven by a children’s roundabout. The idea was that the kids could exercise, have fun and provide safe water for their villages at the same time.

It looked ingenious and donations flooded in. After a while, however, it became clear that the system was very expensive and inefficient. Unlike with normal roundabouts, pushing these was hard work and the children tired quickly; worse still, they sometimes fell off and injured themselves. Ordinary hand pumps were better in almost every way and cost a quarter of the PlayPump’s $14,000 each.

We have heard so many stories of misguided projects and misspent money over the years that surely the time has come to demand evidence that the charities we ­support are effective. But how do you measure whether a charity is effective? One answer would be to apply two tests: does it achieve its stated goal and does it do so as cost-efficiently as it can? A charity such as Guide Dogs might pass this test. But for effective altruists, in deciding whether to give to Guide Dogs, you ought to ask another question: could you get more altruistic bang for your buck by giving to something completely different instead?

They say you can. Guide Dogs UK says it costs £32,400 to train a guide dog and its owner and then another £12,800 “to support the working partnership”. In contrast, Singer says you can save someone from going blind in the developing world for between $20 and $100. “If you do the maths,” he writes, “you will see that the choice we face is to provide one person with a guide dog or prevent anywhere between 400 and 2,000 cases of blindness.”

For effective altruists, the question we should ask is straightforward: what kind of donation will result in the greatest increase in the number of quality-adjusted life years (qalys)? This is a more sophisticated version of the question of how many lives a donation will save. It’s Jeremy Bentham’s old utilitarian principle of the greatest happiness of the greatest number updated for the age of cost-benefit analysis. And this implies that all sorts of charitable causes just don’t deserve your money. Forget universities, art galleries, guide dogs, even cancer treatment and hospices. Instead, focus on insecticide-treated net distribution to fight malaria, deworming of children in Africa and Asia, or giving direct transfers of cash to extremely low-income households.

There is something both irresistible and intolerable about this logic. On the one hand, who could argue that it isn’t better to use a sum of money to save a life rather than merely improve another? But on the other hand, it seems harsh to ignore the needs of everyone other than the very worst-off.

I think this tension points to a grave problem with the rationale for effective altruism. This is best illustrated in the example Singer gives of Zell Kravinsky, a philanthropist who has not only given money but also donated a kidney to a stranger. Kravinsky reasoned that the risk of dying by becoming a donor is one in 4,000, so if he did not donate his kidney he would be valuing his own life at 4,000 times more than that of a stranger, which he thought indefensible.

Kravinsky is thinking like a good utilitarian. But it is one thing to say that, objectively, all lives have equal value. It is quite another to say that the only truly moral way to live is to take an objective perspective and to care equally about all lives. Indeed, in many ways we might think this would be inhuman. What kind of person cares just as much about a stranger as they do about their partner, their children, their friends?

Singer accepts that “there are good, broadly utilitarian reasons” to encourage close relationships. He believes it is good if people say: “Of course I care for my spouse and children more than I care for strangers, but not by a factor of 4,000.”

There is something bizarre about even trying to compute what the right factor is. It’s like the couple Singer describes deciding whether to have children, who worried that it would be immoral to do so because it would take up too much time and money. In the end, they concluded that there was a good chance their child would do more good than harm and this would help “offset the costs of raising them”.

The trouble with all of this is that it establishes, as an ideal of human living, a kind of disinterested detachment that makes us look at the world as though it were a machine for producing happiness, our job being to make sure it functions as efficiently as possible. If we take this role seriously, we are all obliged to give away almost everything. You might forgo a fancy meal in order to give to feed someone who can’t eat, but that still leaves many other things that are comparative luxuries. Your comfort in having the heating on, for instance, is nothing compared to someone else’s discomfort in being malnourished. So there always seems to be a case to give away more. Where does it end? “The only ultimate end,” Singer tells me, “is the point at which if you give more you will be making yourself worse off by a margin that equals the amount that you’re making someone else better off.” Singer calls it “the point of marginal utility”.

This would require us to give up all the higher goods of life, such as art and culture, and live as though survival were the only goal worth pursuing. Singer bites the bullet. “I would be delighted if there were a world in which everyone could have that rich kind of cultural life, but I think at the moment, when you have a billion people in extreme poverty, there is something indecent about saying that you think it is more important to renovate the concert hall at the Lincoln Centre than it is to help people get enough to eat or prevent people going blind.”

The whole purpose of meeting our basic physical needs is so that people can go on to meet their non-material needs. Of course we want people healthy and out of poverty. But when we give people that opportunity, they do what everyone else does: they try to live a full life. They do not keep themselves at Singer’s point of marginal utility.

Then there is the argument that giving up all our relative luxuries might be counterproductive. In accepting that earning to give can be the best way to make the world better, effective altruists are acknowledging how important it is that the wheels of the capitalist economy keep turning. It is widely accepted that economic development has done more to raise people out of poverty than aid, and Bill Gates wouldn’t have billions to give away unless lots of people were buying computers. Effective altruists accept that the poor are best served by economic growth, combined with better governance and fairer trade. Yet they tell us to stop spending on the things that contribute to this growth and give money away instead.

Singer doesn’t see a problem. Sure, we might get into trouble if everyone stopped spending overnight, “But that’s not going to happen. If we start to see that the economy is declining because people are giving so much, the first question would be: is this matched or outweighed by the improvements in the economies where more aid is helping, more people are getting educated and getting jobs? If it is, then I’d say, from a universal perspective: ‘That’s a good thing.’ If it’s not – if our economy is sinking and it isn’t producing that kind of improvement in the position of the worst-off – certainly at that point you’d want to stop and say, ‘Wait a minute. Maybe this has gone too far.’”

This response undermines the case for saying that giving to the point of “marginal utility” is a basic obligation. It acknowledges that the few who give away so much are effective only because most people give so little. They are compensating for the faults of others, going beyond the call of duty and acting heroically. This is exactly what we think about people like Ord, whom we admire for doing so much. But oddly, although we praise them for their exceptional efforts, they blame themselves for not even getting close to the point of marginal utility.

When I suggest to Singer that his is a moral principle that is not fit for human beings, he turns it around, saying: “It’s a moral principle and human beings are not fit for it. If we were better beings, more moral beings, that is how we would act and I don’t think it detracts from the validity of the principle that we have evolved through a process that makes us inevitably self-interested to a significant degree.”

I’m not so sure we would be better people if we gave our own welfare and that of those we love no more importance than that of any other living creature. In some respects it would make us more compassionate, but in others it would make us strangely detached, more like gods looking down on earth than human beings living here. We would be focused only on achieving good goals, when life is at least as much about how we live it as where we end up. Earning to give is to treat your life as a means to an end, not as a valuable end in itself. Effective altruism challenges us to do more and to do it more effectively, and if that moves us in a more altruistic direction, that should be welcomed. But what it lacks is a credible ideal of what the good life we are working towards looks like.

There is another, completely different problem, however, with the rise of effective altruism. We are living at a time when many are looking away from the state in trying to solve our deepest problems. The likes of Singer rightly reject the notion that there should be any either/or here, arguing that we need both individual giving and political change to solve world poverty. Yet in practice, a greater emphasis on private philanthropy usually goes hand in hand with less trust in political solutions. The root causes of poverty are structural and not founded on the failure of individuals to give more. Effective altruists are right that we all could and should do much more. But as even they accept, we should be careful not to allow a focus on the most good we can do individually to distract us from the much greater good we could do collectively.

Julian Baggini’s latest book is “Freedom Regained: the Possibility of Free Will” (Granta, £14.99)