The idea of rationing medical treatment is one we instinctively find difficult. The National Health Service provides an ever-increasing range of life-saving and life-prolonging treatments, many of them unimaginable to previous generations. That in itself has compounded the problem: an ageing population needs more medical care.

Providing the best treatment for everyone on the basis of need is one of the ideals which unites British society, but it is not universally applied. The health service is run separately in Scotland and England and since devolution the divergence has been significant. One of the most obvious examples is in the prescribing of new drug treatments, such as Herceptin for some breast cancer patients. Most Scottish health boards say they prescribe it in appropriate cases, while English patients have taken legal action to obtain it. We have recently seen a similar discrepancy north and south of the border in the prescribing of Alimta for mesothelioma.

We cannot - and must not - assume that such a generous policy will continue indefinitely in Scotland.

In England, the British Medical Association tomorrow publishes a paper warning that fertility treatment, plastic surgery and more minor operations such as those for varicose veins and glue ear are likely to be rationed to the point where they may no longer be available on the NHS. Doctors in Scotland also concede that it is no longer financially feasible to deliver everything to everyone.

The truth is that there has always been rationing in health provision and there are always difficult judgments to call. Physicians and surgeons must not only decide whether operations and drug treatments will be effective without causing undue suffering, but also weigh the economic cost against patient benefit.

As taxpayers and patients, however, it is time we all recognised emerging realities. No matter how much money is allocated to the NHS, there is always a case for more. As Dr Andrew Walker, health economist at Glasgow University, points out, we are going to face a growing mis-match between available resources and the demands placed on them. We can no longer hide our heads in the sand. The open discussion called for by the BMA in Scotland will not solve the problem, but it will be the first step towards more rational policy making. Above all, public discussion must prevent any political hijacking of an issue that is vital to us all.