The past month has seen two bitter rows about education. The most prominent, the Battle of St Olave's, was a heady cocktail of selection in schools with a gripping subplot about hypocrisy and about just how far the personal is political. The second was about universities and the Committee of Vice Chancellors and Principals' recommendation that students should pay Pounds 300, albeit temporarily, to bridge the gap between rising numbers of students and static funding.

Both rows are likely to continue beyond the next election for two simple reasons. One is that education is seen as the commanding height of the economy and society, the source of power, status and success. The second is that both rows are about fairness at a time when we seem to lack a workable consensus about what is fair.

Sometimes fairness is discussed solely in terms of whether people are prepared to pay for redistribution through the tax system. But this misses the point. While philosophers have tended to interpret fairness in terms of social justice, the public takes a different view. It is certainly unfair to let anyone starve or be homeless. But fairness also means fair rewards. So if someone works hard, or is clever or creative, then it is only fair that they should be rewarded. This is why the public is entirely happy with Richard Branson and Madonna accumulating vast worth, but far less happy about the chairmen of monopoly utilities, where it is hard to see any direct link between merit and reward. It is also why there is such resistance to benefits systems that appear to reward idleness.

In the past social theorists used to claim that any problems with fairness could be resolved with reference to a metaprinciple of social justice. But in practice justice is never as simple. All of the empirical studies of public perceptions of justice and fairness have shown just how distant they are from the philosophers' grand designs.

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The problem in education is that so little has been done to articulate viable rules of fairness that would make policies work but also be legitimate. We lack adequate thinking about whether it is fair to pay for schooling or to subsidise professional qualifications. The one-dimensional legacies of both the old left and the new right have made it impossible to admit that perfectly valid principles of fairness and equality may conflict.

If egalitarianism is pursued more vigorously in the state system, more people will opt out altogether and buy a better education (especially if, as at present, this cannot be legally prevented). The practical effect of a commitment to equality may be to increase inequality, whereas increasing selection in the state system may be more likely to include the middle classes more and thus foster social cohesion.

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If the education system is not to be subject to continual waves of not wholly legitimate reform, we need much greater clarity. However hard it may be, it is possible to define some of the limits to what is fair. For example, most would agree that it is unfair to prevent some differentiation according to merit and that it is unfair for parents to be able directly to purchase privilege for their children through a private schools system.

Most would agree that so far as universities confer lifelong advantages on those who go through them, this should be balanced by a matching obligation to pay. But these are just starting points. The challenge is for the educational theorists to prove that they really can take us beyond the easy rhetoric of choice and equality.

Geoff Mulgan is director of Demos, the independent think tank.