Iain Duncan Smith made his political name as a Eurosceptic, revived his career in opposition as a poverty-fighter, and has always been a social conservative – believing that strong families are the foundation on which a strong society is built. But at the core of his thinking as Work and Pensions Secretary since the start has been a belief in the life-transforming power of work.

For the overwhelming majority of people, work is the only practicable route out of poverty, and the means of making the money without which couples can’t stick together.  So it is work that binds all his main political interests together – even, up to a point, the European one, since it is epic rates of unemployment in Greece and some other Euro-member countries that threatens their political stability and the peace of our continent.

But for the Work and Pensions Secretary, work isn’t simply a route to reducing the cost to the taxpayer of subsidising unemployment, a way of strengthening family life or even a means of reducing poverty (and across generations, too).  For him, even more importantly, it is fundamental to human well-being.  People who don’t work are more likely to suffer from anxiety, depression and mental illness.  “Work is good for health,” he will say in a speech today. “Growing evidence over the last decade has shown work can keep people healthy as well as help promote recovery if someone falls ill.  By contrast, there is a strong link between those not in work and poor health.”

The speech goes on to set out much that he and his fellow Ministers are already doing: in particular, the Work Programme, which has moved over 430,000 people into lasting employment, and the fledgling Fit for Work Service, which is focused on those with mental health conditions.  In essence, a doctor has little option, if a patient comes to him with such an illness, other than to sign him off work.  Duncan Smith is sharp in his speech about employers who simply allow their employees with such conditions, or who are sick for other reasons, to languish on sick pay and then sickness benefit – or who shy away from hiring disabled people in the first place.  (He doesn’t quite say that “British businesses need a kick up their lazy arses,” as one Cabinet Minister is reported to have said at the time of the budget, but the sense is there.)

This is where Fit for Work comes in.  As the Work and Pensions Secretary puts it, “instead of asking ‘how sick are you?’, the new service asks, ‘what help can we give you now that will help and keep you close to your job?…our Fit for Work service includes professional experts skilled in helping people with mental health conditions. That is why this Government is investing in psychological treatment services which are helping thousands of people return to work from a period of sickness absence. And that is why we are also investing and testing new ways of joining up health and employment services to improve access to treatment and support.”

But there is a limit to what Fit for Work can do, even after it is rolled out more widely.  For the stubborn fact is that while the Coalition made a real impact on unemployment in general, it has made less on some in particular – in particular, that which blights the lives of disabled people, most of whom want to work (as disability groups used to stress to me when, in opposition, I was Shadow Minister for Disabled People).  As the Sun (£) reports today, “while the number of people on the dole has nearly halved since 2010 – falling by close to 700,000 – the number of incapacity claimants has fallen by just 90,000. The employment rate for disabled people is 33 per cent lower than that for the general working age population.”  And in Duncan Smith’s view, the reason why it remains so stubbornly high, and why Fit to Work is less effective than it could be, is all bound up with the way the benefit system works.

Labour restructured incapacity benefits to create an Employment Support Allowance (ESA).  The intentions of James Purnell, who undertook the reform as Work and Pensions Secretary, were the same as his successor’s: to get more disabled people into work: so far, so good.  But its expansion – undertaken by Yvette Cooper, as Duncan Smith points out (in a side-swipe at the current Labour leadership contest) – went badly wrong.  In simple terms, ESA asks whether a person is available for all work or incapable of all work, and not whether he or she can do some work, perhaps part-time.  This will not be the case with Universal Credit, into which ESA is being rolled.

The core of the Work and Secretary’s speech, therefore, is his proposal to bring the two into line so that we have, as he puts it, “a system focused on what a claimant can do and the support they’ll need – and not just on what they can’t do.”  He wants to crack the latest manifestation of a problem that has haunted it since Britain’s economy changed from from one concentrated on manufacturing, sustained by jobs for men, to one in which services and working women play a far bigger role.

“During the 1990s, the key problem was bad backs and, today, it is mental conditions,” a Minister told me.  This was true in the era of the Blair and Brown premierships, not just in this new era of Conservative majority Government.  I remember being struck by the exponential growth of claimants with mental illnesses when getting the information out of Labour Ministers via written questions.

Work isn’t everything.  As Duncan Smith acknowledges, some people of working age will always have illnesses that leave them incapable of any work.  Furthermore, others shouldn’t be in the labour market at all if they don’t want to be – mothers or fathers with very young children, for example.  The Government’s childcare policy doesn’t meet their needs at all.  And for those who are employed, work should be balanced with leisure: we work to live, not live to work.  This is why ConservativeHome opposes further relaxations of the Sunday Trading restrictions.

Whatever reform is made to ESA, much depends on how Fit to Work, and similar programmes, are implemented.  If Labour’s leadership contest leaves it in any condition to carry out effective opposition at all, it will search far and wide for horror stories of bungled assessments, and dump them on the Work and Pensions Secretary’s doorstep.  But the thrust of his argument is incontestable and, as polling evidence keeps finding, very much in touch with what younger voters in particular believe about the future of the welfare system.

Yes, that balance of work and leisure is good for you, and Duncan Smith’s pursuit of the former is helping to drive Britain’s jobs miracle – which sees the number workless households at a record low: it doubtless pleases the Treasury, too. Terrier-like, the Work and Pensions Secretary has got his teeth into worklessness and, pull or push him as you like, blame him or praise him as you will, he simply won’t let go.  Nothing will stop him.  It is this tenacity that has sustained his mission and brought him back from political oblivion, and justified David Cameron’s decision to put him in his own present job.

“Labour is the one thing a man has had too much of,” wrote D.H.Lawrence.  “Let’s abolish labour, let’s have done with labouring!/ Work can be fun, and men can enjoy it; then it’s not labour.”  The poem from which those lines are taken was titled “A Sane Revolution”.  He wanted to “have it for fun”.  Duncan Smith is deadly serious.