David Cameron is right, though only up to a point: many people in this country are offended by the thought of workers in poorly paid jobs having to support benefit claimants who can't work because of addictions to drink, drugs or food. Even some politicians on the centre-left have come round to recognising the inherent unfairness of the present system, although I suspect they may have winced when they heard the Prime Minister's language last week. Some of his Lib Dem coalition colleagues certainly didn't like it.

Cameron is still trying to present himself as a compassionate Conservative. But the revelation that just over 80,000 people are living on incapacity benefit because of alcohol, drugs or weight problems sent him into a classic Tory rant. He promised to get tough with claimants who don't deserve benefits – the "undeserving poor" are a perennial Tory theme – and he was clearly addressing his core vote when he talked about the need to get "these people" into work. What he didn't explain is where the new jobs are going to come from, just as he failed to address the question of why employers would take on alcoholics or drug addicts (to put it crudely) when there are 2.5 million unemployed but reasonably healthy potential workers to choose from.

The PM's error was to correctly identify an imbalance in the treatment of taxpayers and benefits claimants, and then make a leap from that into punitive moral judgements. He also gave the impression that the problem is bigger than it is, even though fewer than one in 20 incapacity benefit claimants fall into this category. According to the Department for Work and Pensions, in August last year there were 42,360 incapacity benefit claimants with alcohol addiction, 37,480 with drug dependency and 1,800 who were obese. Labour's spokesman on employment, Stephen Timms, pointed out that there's a danger of simply moving thousands of claimants from one benefit to another; if they can't find work, they may end up on Jobseeker's Allowance and nothing will have been achieved.

A complex problem that has developed over years isn't amenable to the simple, instant solutions that politicians favour. The Employment minister, Chris Grayling, says private and voluntary organisations have agreed to invest £580m in treating addicts and preparing them for employment. Modern Tories like to think themselves caring, but their instincts are still small-state – and he claims all conditions are treatable with the right support.

I wonder if he's ever met some of the families I encountered while canvassing in the north of England during last year's general election. One couple in particular has stayed in my memory, their problems so intractable that the woman broke down in tears when I arrived with some leaflets. Her husband looked to be in his early forties but stayed on the sofa in a dressing gown, so obese he was unable to get up without help. He had diabetes and was waiting for two new hips, while his wife had had to give up work to look after him. Both were severely depressed, entirely dependent on state benefits and unable to see any way out.

They were both very nice people, living in a way neither had consciously chosen, although in strict Cameronian terms the husband had brought his problems on himself. It would take years and massive state intervention to get this couple into a situation where they could work and support themselves, and that takes us to the problem at the heart of Tory philosophy. Time after time, the most significant factors in what happens to people turn out to be class, education and income, but Tory ministers behave as though we all have the same start in life. This inevitably colours their view of individuals, encouraging the kind of unfeeling language we heard last week.

Take obesity, where a gap is opening up between children from poor and affluent families. Some researchers suggest working-class children are fatter because their families aren't getting messages about healthy eating and exercise; others point out that fast food is cheaper, less time-consuming, and more filling than a healthy middle-class diet. Overweight children tend to grow into obese adults, but Tories, as a breed, have little appetite for regulating supermarkets which discount unhealthy food and alcohol.

I know alcoholics can be irritating and self-deceiving, but cheap booze, social tolerance of heavy drinking and the reluctance of GPs to intervene all play a role in failing to halt addiction at an early stage. Cameron's Tories seem less interested in understanding why people develop addictions than finding fault, labelling individuals and telling them to get over it. Weirdly for people who loathe the "nanny state", they're pretty quick to sound like nannies themselves, berating their charges for indolence and fecklessness. "Frankly," Cameron fumed, "we are not doing our job looking after taxpayers' money if we do not try and make sure these people go to work."

Right-wing politicians have been talking like this for years, pleasing their voters but failing to come up with long-term answers. Labour governments make a different mistake, reluctant to stigmatise the poor, but also seeking to avoid antagonising constituents whose families depend on benefits. Political parties disagree about whether incapacity benefit claimants are shysters or victims, but their responses are too tribal to address the question of fairness for everyone: people in work, the unemployed, pensioners, the chronically sick.

Cameron is right to argue that too many people are "trapped in long-term poverty", but that doesn't apply only to people on state benefits. Inequality is entrenched, and you don't have to look far to find the hopelessness, low self-esteem, poor health and multiple dependencies that are among its consequences. Getting people out of this state requires radical rethinking about the relationship between rich and poor, individuals and state. But it might put an end to the absurd situation in which the poorest exist on the taxes of the not quite so poor.

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