The belief that we are naturally and fundamentally selfish, from our genes upwards, may be the most powerful of all the acids eating at the foundations of the welfare state and of the wider postwar liberal order. It gains its power from the fact that it is half true. It gains its danger from being half a lie. Everyone but the most miserable knows from personal experience that people are full of goodness as well as of its opposite. We are fairly generous, unselfish, even sometimes thoughtful and trusting in our private lives, most of the time. Public life, however, is increasingly conducted as if universal selfishness defined human nature, and politics must be a zero-sum game. In countries where the economy is stagnating, as Britain’s has been for the past 10 years, this is horribly credible. Each pound in lower taxes for the rich is taken from health or education for the poor; security for the old (if they’re lucky) is funded by vastly inflated house prices and insecurity for the young. The interests of immigrants are set against those of the indigenous population. The interests of Britain are set against those of the EU.

None of these oppositions are inevitable. Groups need not be selfish any more than individual people have to be. What most of all makes them so is mistrust. One of the motors of distrust and meanness in ourselves is the belief that others are cheating. The belief that the others are in it only for themselves is at the root of almost all assaults on the welfare state, and even on the ideology of welfare. It contributes to the gleeful trashing of the reputation of charities. Acts of breathtaking selfishness like the Republican tax cut – which has just awarded $29bn to the profits of one firm alone, Berkshire Hathaway, controlled by one of the world’s richest men, Warren Buffett, at the expense of healthcare and even food for millions of poor Americans – are justified and made politically possible by the belief that the poor are endlessly grasping and can never be given enough: so why not give them nothing.

The consequences have been grim. In the words of the archbishop of Canterbury, Justin Welby, in his forthcoming book, Reimagining Britain, on ways out of the crisis, “Austerity is not merely an economic term. It … almost invariably conceals the crushing of the weak, the unlucky, the ill, and a million others. Austerity is a theory for the rich and a reality of suffering for the poor.”

But it is easier to diagnose the crisis than to react to it. In any case, preaching would be a useless response. Fortunately that is not what the archbishop is up to in his book. He is realistically, rather than theologically, concerned with human weakness: there is more Hobbes than Calvin here. Nonetheless, his argument is clearly derived from Catholic social teaching, something that started with the attempt to join Aristotle’s psychology with a Christian view of our purpose in the world. So it was from the beginning an attempt to graft a universalist ethic on to a view that took inequality for granted and actually saw it as a essential for civilisation. The conflict between those perspectives still rages today, within our hearts as well as our communities, within churches and within secularist movements too.

The answer proposed by Catholic social teaching, and presupposed by most progressive thought, is that we should be working toward the common good. This is distinct from the fulfilment of individual wants, and will sometimes be opposed to them; but in the long term it may be the only way to satisfy our partially selfish and partially altruistic natures. What’s new in contemporary developments of this thought is its accommodation with pluralism. Instead of one common good for one ideal community, there may be many communities, none of them actually ideal. Such an idea allows us to be members of many different communities at different scales simultaneously. There is no one grouping, whether nation, family, or global village, that should command all our loyalties; they must all negotiate and compromise with each other, just as we must negotiate and compromise within them. This is not just morally better than universal selfishness. It is more realistic, too.