Had Theresa May not on Monday summarily abandoned her manifesto threat to raid the savings of those who end up senile in care homes, I had planned to defend the idea here in terms that might have added to her woes.  I’ll do so regardless. The so-called dementia tax would, over time, have become a euthanasia bonus. And that would be a good thing.

As I argued on this page two weeks ago, morality is the father of religion, and not the other way around. Secular morality can be largely explained by social Darwinism. For a society to prosper it requires an ethical framework that boosts, rather than encumbers, the tribe’s chances of survival; this explains previous or current taboos on (for example) homosexuality, suicide, incest or abortion, as well as ethical premiums placed on marital fidelity, ‘family values’ and civic responsibility, and the virtues of heroism in battle. Religious faiths latch on to these essentially secular values and clothe them in the garments of piety; and because in so doing such ideas are invested with divine authority, the church (or mosque, or synagogue) often gets stuck with rules that have become harmful, dragging its feet as secular morality moves on.

A difficulty, however, that both religious and secular morality share arises from what genetic science calls ‘spandrels’ (a term for small, circular architectural features, often decorated, whose insertion arises from the fact that if you support a platform upon an arch, you end up with curved/triangular spaces to the left and right of the top of the arch). Spandrels may be incidental to the structural engineering but have come along for the ride.

‘Thou shalt not kill’ has landed society with some awkward spandrels. The injunction is obviously in a society’s interests: murder is socially harmful. But what happens if, for its own defence, a society has to engage in war? Morality, both secular and religious, has over the millennia learned to shrug, and explain that ‘thou shalt not kill’ doesn’t always mean that thou shalt not kill. Indeed, sometimes thou shalt. But it’s awkward.

Awkward, too, in the case of lives that have become a great burden on society. We have on the whole declined to make any other big exceptions to the commandment (beyond war) because the question of who decides is so difficult, and the burden has always been bearable. In the case of those whose advanced senility means they can bring neither happiness nor usefulness, even to themselves, the burden has been limited by the fact that ‘nature’ has tended to end these lives before too long anyway. As the kindly but candid senior nurse, surveying the pitiful ranks of helpless, hopeless, senile old ladies in her care, once put it to me on my Christmas visit (as MP) to a rural nursing home in the Peak District: ‘In the old days, a good Derbyshire winter would have cut through this lot like a knife through butter.’

That is no longer the case. We have conquered nature at least to the degree that we can prolong life for decades — even if it is not an active, wholly sentient life. The burden this is placing on our economy, on family life, on state spending, and on our health service, is growing fast and relentlessly.

Will our society’s innate Darwinian will-to-survive begin its work of slow, subliminal influence over our moral sensibilities, then? Will social attitudes shift, so that while still sharp, active and fit, we feel a dawning obligation to make provision for our own deaths when the burden on family and friends becomes intolerable? Will the medical profession move more decisively in the direction of helping to expedite this? The answer to these questions is that something is delaying this natural evolution of secular morality. In many cases, much (sometimes all) of the financial cost is not falling upon the individuals with family ties to those suffering from senile dementia. To put into words what I mean, I have been trying to find a way that did not sound brutal and perhaps flippant, but the truth is brutal. Where the state is largely or wholly responsible for the care and cost of an elderly person’s dementia, no individual has an overwhelming interest in their timely passing. If the state pays for care — often for a decade or more — and upon death the surviving family inherit a legacy that is undiminished by the huge cost of that care, what is it to them that the life has been unnaturally prolonged?

Nobody wants to think about, let alone decide, how long someone they have loved should live when their life has become meaningless. When the cost of life falls upon the general taxpayer, why argue against life? When I am old, and if I then suffer from dementia, it would be painful (I hope) for my (younger) partner to put into train anything that allowed, or helped, my life to end. The easiest thing for him to do will be to put me in a nursing home, visit me dutifully occasionally, and leave the taxpayer to pay the bill — with all my estate headed safely his way when I go. If a tough and harrowing decision can be avoided by sending the bill to the Chancellor of the Exchequer, I’m afraid that’s where the bill will go.

Please understand that I’m not suggesting that if after 8 June the cost of dementia care fell heavily upon millions more fellow citizens, millions more would wake up on 9 June and declare they no longer believed human life was sacred. I’m saying that as the bills for dementia care thudded more heavily upon the doormats of those who hope to inherit, then very, very gradually, probably over generations, the argument for letting or helping people die when their lives had emptied would begin to find more favour.

Pooling the cost is retarding that process, but not reducing the cost. This manifesto was a chance to bring that cost home to individuals. I’m sorry it will now be missed.