In Monty Python's Life of Brian, at a meeting of the People's Front of Judea, Stan declares that from now on he wishes to be known as Loretta. He wants to have babies. “You can't have babies,” exclaims Reg, the party leader. “You haven't got a womb. Where's the foetus going to gestate? In a box?” “Don't you oppress me,” snaps back Stan/Loretta.

Then Comrade Judith puts forward a compromise: “Here. I've got an idea. Suppose you agree that he can't actually have babies, not having a womb, which is nobody's fault, not even the Romans', but that he can have the right to have babies.”

And then, when she had finished mediating between members of the People's Front of Judea, Comrade Judith sat down and started to work on a constitution for the National Health Service. Or at least the document published by the Government on Monday reads as if she had been involved.

The rights established by the new draft constitution for the NHS can be neatly divided into two - those that you already have and which are not changed by its publication, and rights that it suggests it is providing you, but that in truth you do not have and will never have. For reasons that are no one's fault. Not even the Romans'.

It is a document whose publication might have been excusable when the NHS was founded 60 years ago, but is not excusable now.

In 1948 the founders earnestly believed that the costs of the new service would go down as universal care improved national health. Promising, as the new document does, “a comprehensive service, available to all” and free of charge would have seemed at the time perfectly possible and sustainable. To use such phrases now, after all we have learnt about rising costs, is like promising Stan his right to have a baby.

We all, deep down, know the score, don't we? A technological revolution in medical services is under way. It means that we are going to reach the point, are already reaching the point, when the range of services we could, in theory, obtain is so great that it tests our willingness to pay for it all for everyone. We are going to have to decide where the limits of social provision lie.

We all really understand this, but we try to dodge the consequences of what we know. The first dodge is to pretend - as, by omission, the NHS constitution does - that this decision on the limits of the NHS is still a long way off and not one we need to grapple with yet. This won't wash. The decision is with us now.

There are more than 40 new cancer drugs in the final stages of development - the products of the molecular revolution triggered by the discovery of the structure of DNA. These work. They extend lives. But they are expensive - the cost of getting a single drug to market now exceeds $1 billion. So, many of them will not be available on the NHS. How can this just be ignored?

How could the NHS constitution pass over it in silence? The reason why it does so is simple. On its first page it boldly, but extraordinarily, announces that the NHS “has a wider social duty to promote equality through its services”. It cannot therefore acknowledge a reality that makes complete equality impossible. Yet refusing to acknowledge reality does not make it go away. As the oncologist Professor Karol Sikora puts it: “Denying the existence of innovative drugs is no longer acceptable in a democracy where patients can have access to all the information.”

The second dodge is to pretend that defining the limits of social provision need only mean excluding tattoo removal and other cosmetic procedures. The problem with this dodge - common though it is - is pretty obvious. The amounts of money such a rule would involve are piffling. Not least because most primary care trusts already refuse to carry out such procedures. I have a long list of such treatments in front of me, sent by one PCT to its practitioners to ensure they don't carry out any of them without further detailed consideration.

And then there is a right-wing dodge, a dodge favoured by those posing as radicals. This is to argue that replacing the NHS with something airily referred to as “social insurance” (without being too precise about what that actually means) will magically make the problem of funding new drugs go away. It will not. In the end, whatever the exact form of social provision, the line will still need to be drawn between those things we agree to provide to everyone and those things individuals will decide to purchase (or be insured for), depending on their income or inclination.

Does it matter that we muddle along, rationing by stealth, never being too clear? Yes, it matters a great deal. The rules that govern the dispensing of drugs that are not approved by the National Institute for Health and Clinical Excellence lead to a great deal of stress, as families fight for life-saving treatments using unclear guidelines and bureacratic procedures. There is a vast amount of variability and inequity.

The refusal to allow patients to pay for these treatments while continuing to use the NHS has been a scandal. (Was the NHS constitution really silent on this? Oh yes, it was.) Yet if so-called co-payment is allowed, there will be a need for further rules: who pays, for instance, for medication to deal with the side-effects of privately purchased drugs?

So why doesn't the Thomas Jefferson of the Department of Health open a new Microsoft Word file and get going on a new declaration? What is required is a quite different document. Not one that announces all the rights you have and all the things the NHS promises to provide you, but one that does quite the opposite. A document that tells you all the things you don't have a right to and the NHS refuses to promise you. Now that would be a constitution worth having.

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