Alan Milburn is fond of reminding people that the local council chose the colour of the door of the house he lived in. The story neatly encapsulates both his rise from a council estate to the Cabinet and the witless way in which a local bureaucracy can stand in the way of aspiration. If it is true, as Mr Milburn says, that the journey of his life is less likely for young people born today, then this is a fact to be regretted.

This basic idea - that rewards should accrue to merit - goes under the political code of social mobility. Today, a White Paper on social mobility is published. On the face of it, this is an odd objective. Mobility is not a principle - a society could very well be mobile without being fair, in much the same way as clothes are mobile in a tumble dryer. And politicians do not like to admit that a society that was both mobile and meritocratic would require the dull offspring of the wealthy to fall down the social snakes at the same time as clever, poor children were climbing the ladders.

That said, this paper is good politics. It is, before it is anything else, an attempt to root the reform of public services in a higher purpose. The damaging critique of the Blair years is not that the reform plans were wrong but that they were always used as a stick to beat the Labour Party with and, therefore, too little was achieved.

The White Paper is also an important sign that there are some in the Government who realise that demanding gratitude for its conduct of the financial crisis is no prospectus for a general election. The latest Populus poll for The Times - which gives the Conservatives a 10 per cent lead - suggests that the patience of the electorate with the Government's frantic activity may be wearing thin.

The paper does not contain detailed policy proposals beyond the headlines about free nursery places and bigger incentives for teachers to work in poorer areas. But this is where the argument gets very difficult. The motor force of mobility in Britain has been a secular change in the structure of the economy. There were more professional jobs at the end of the 20th century than there had been at the start. It is not easy to attribute the shift to policy: countries with very different social policy regimes have very similar records on social mobilty.

Mr Milburn has been appointed to lead a panel to investigate how more children from poor backgrounds can be helped to pursue careers in the professions. It is always tempting to blame recruitment practice and it is true that they are by no means always fair. But the roots of social immobility go back a long way. Mass education has not become the great social engineer that social democrats believed it would. Time and again expansions of the welfare state have benefited the middle class most, the expansion of university education being the latest example.

The prerequisite of social mobility is a wider spread of power. That means money in the hands of individuals, not bureaucracies. It means greater institutional flexibility. It means central government steering rather than rowing. Social mobility is a complex idea that is the upshot of very many individual lives. The risk is that, somewhere in the system, there is always someone with the authority to decide who has a strong view on the colour of Mr Milburn's door.