First, it directly confronts issues of justice and fairness, instead of giving priority to numerical calculations. Of course we need the relevant numbers - for example, the proportions of young people from different social classes who enter higher education from school. These show glaring gaps in attainment. But discussions of equality can too easily focus on attaching numbers to different groups. How nearly should the current student population match the population as a whole in terms of sex, ethnicity, age and disability? Often the underlying assumptions about why these numbers would make an acceptable picture are simply not explored. You can tell from this that I am not a supporter of rigid quotas, other than, perhaps, on a very interim basis.

Second, the notion of equity introduces a dynamism into the debate. Our understanding of fairness changes over time, in relation to educational opportunity as in other spheres. Sometimes it may be because a particularly effective lobby is at work, or a media campaign is running that highlights the interests of a specific group. But we need also to be able to bring into the same equation the needs and claims of people from different cohorts. How do we assess the claims to educational opportunity of the over-65s, who largely missed out on post-compulsory opportunities, and at the same time the claims of the under-fives, who as adults will be entering into a world that we can only guess at?

The practical import of such questions is illustrated by a recent report on the social wage from the Joseph Rowntree Foundation, once again admirably fulfilling its aim of highlighting equity issues. Higher education for non-mature students, it shows, is strongly biased in favour of the top income group. But if we look further along the lifecourse, higher education for mature students has a small but clear pro-poor bias.

Equity needs to be seen in a lifelong context. Using “equality” as a basic concept too quickly runs into the kind of problems that racecourse officials had before starting stalls were introduced, trying vainly to bring all participants into line before lifting the tapes.

The difference is that for education there is no tidy finishing line - and not everyone is running the same race.

What we have is a continuous struggle to distribute opportunities fairly, not just between groups but also over time. The distribution affects us all, in ways we can only guess at. By any standards we have a high output of very well-qualified graduates. But the potential value of this asset is diminished because we also have a large number of people who never achieve educationally throughout their lives. In one sense, this puts the high achievers in an even stronger position.

Although no job today is secure, they will have far the best chance of holding on to a job, or moving to another job of similar or better ranking. The most recent evidence shows graduates as far ahead as ever in earnings. Interestingly, this is the case even with more women than men - the gap between female graduate and non-graduate earnings is even greater than it is for men.

But the highly qualified are also harmed by others’ lack of a general level of attainment. The success of their organisation is held back by the low level of basic competence. The fruits of their success are less enjoyable because of the social instability which goes along with education failure.

Looked at from this angle, the role of an equitable educational policy is that of a sheepdog. It is to round up the stragglers, chivvying them on and making sure that the flock as a whole stays more or less together.

The metaphor cannot be taken too far - who would want to suggest that students are destined for the pen? But continuous duty to support those at the back seems to me quite aptly captured. In the long run, equity is probably in almost everyone’s interest; this doesn’t mean that there aren’t still hard questions to be answered about what it means in practice.

Tom Schuller is director of the Centre for Continuing Education at Edinburgh University