Watching Robert Kilroy-Silk stirring up a television audience over rationing in the NHS was not a comfortable experience. Presiding over the modern-day equivalent of the Roman arena, Kilroy-Silk pitted doctors and other healthcare professionals against 'deserving' patients unable to get the operations they needed because of the iniquitous processes of prioritisation. The audience applauded and booed in all the right places, and at the end of the stage-managed bloodbath all one's sympathies were with the unfortunate patients.

But a telling point was made for the NHS: a patient waiting for a heart and kidney transplant was asked by a beleaguered medic: 'OK, so if you don't like the present system of rationing, how would you choose between a man of 40 and one of 80 awaiting the same operation when only one could be performed?' The question went unanswered.

What we witnessed, though, was much more than a school-room balloon debate where no one really has to jump from the basket to their doom. Healthcare rationing means some people actually do die while others, more fortunate, live.

This kind of rationing dilemma is not new: there are well-documented cases of ship-wrecked sailors adrift on the ocean drawing lots to determine who should be killed and eaten to preserve the lives of the remainder. In Eskimo society it was once taken for granted that, in extremis, elderly people would willingly sacrifice their lives and bodies for their families if they should become trapped by severe arctic weather conditions.

Rationing in the NHS is, in principle, no different and no more sophisticated than this. The underlying assumptions are identical: some lives are worth relatively more than others and should be saved at the expense of those that are less valuable. The need to make such decisions will always exist: the real debate can only be about the criteria for selection.

But it is by no means self-evident what those criteria should be: a system of value judgements is required, and there are many to choose from. Should the rich get priority over the poor? There is a good argument that they should: the rich earn more and pay more taxes, which will support the NHS in future. A rich man's future taxes may save the lives of many; saving the life of a poor man will save only his own.

And what about a rich woman - or indeed all women? Should they be given priority over men? Why is there that traditional rationing cry of 'women and children first' when a ship is about to sink? The answer to that, at least, is obvious - women, who can bear only one or two children at a time, are a scarce and hence valuable commodity, whereas a single man could father thousands of offspring. It only takes one lucky mariner to make it to the shoreline to keep the race alive.

But why stop there? Some social and ethnic groups have higher birth rates than others, thereby contributing more to the future of our community. All other things being equal, wouldn't it be logical to give individuals from those groups higher priority than those from less fertile backgrounds?

Bringing children into the equation raises the question of age. Should children be given priority over their elders? Every mother's natural instinct is to say yes, but then relying on a mother's instinct is perhaps not the best guide to such difficult decisions. How many husbands faced with the stark choice of saving either a wife or a new-born baby would deny life-saving treatment to their spouse - one can always make more babies.

In the real world, too, a society plunged into a crisis such as a civil war soon makes its true collective priorities evident - children are always the first to starve.

At the other end of the spectrum we have elderly people. In Britain, old people currently get a bad deal. Faced with a choice of giving treatment to a 40-year-old or an 80-year-old, the youngster wins every time: but should it be so? Elderly people can legitimately argue that they have pre-paid for their treatment by a lifetime of National Insurance contributions, their accounts are full and so they deserve priority. And shouldn't deference and preference be given simply by virtue of age anyway - surely the older one gets, the greater reverence one should be given as a reward for sacrifices already made?

Even if age alone is not a sufficient priority, what about wisdom? Could anyone argue that an aged Albert Einstein would not be worth more than a young and feckless drop-out with a sub-average IQ? In one year of extended life an Einstein would certainly do more good than the young dead-beat would be likely to achieve if he lived for another 100 years.

'Of course, such arguments are wholly subjective,' many will rightly argue. But is that a useful criticism? In the UK, quality-adjusted life years - or some variant of them - are the preferred method of making rationing decisions. But QALYs, too, are entirely subjective - they merely codify a current preference for favouring the needs of the individual above those of the community; the method is neither more nor less valid than other systems of rationing, no matter how odd they may seem. A simple lottery would be just as ethical.

There is no such thing as an objective method to ration healthcare, and those who believe there is are chasing a mirage. The only morally valid method is that which the majority in any society, at any particular time and in any particular situation, generally believes to be acceptable - and curiously, one way to reach a consensus on that vital question is to keep debating the issues on such irritating programmes as Kilroy.

Steve Ainsworth is a former primary care manager.