To imagine is to understand

There is something about the notion of performing an abortion on one of two twins in the womb that is peculiarly potent. It is popularly supposed that the nation is fundamentally polarised over abortion, and it is true that fierce views are held by activists in both the pro and anti-abortion lobbies. Yet the vast majority of people are far from categoric on the issue; their arguments are about degree and proportionality and are often tinged with an ill-defined sense of discomfort.

So what is it that gives this latest case such stinging singularity? In practice our society believes that the moral importance of a foetus grows with its size. Hence the unending debate about exactly when the cluster of cells turns into a person. Hence the more recent concern at what age a foetus begins to feel pain. Hence our complex rules on the treatment of frozen embryos, which give them a status less than that of a person but more than that of a piece of human tissue.

Anti-abortionists dismiss all this as intellectually dishonest. Just because an embryo is small and not visually familiar we choose to dismiss it in a way in which we would not dream of ignoring the reality of a germ or bacteria.

Conversely pro-abortionists might ask why the religious absolutism that fuels many of their opponents ignores the richness of much of their own theological inheritance. St Augustine insisted that the soul did not enter the foetus until the 46th day, and St Thomas Aquinas, following Aristotle, argued such "ensoulment" occurred at 40 days for the male and 90 for the female.

What the case of the single twin abortion shows is that such moral reasoning is not the entire basis on which our attitudes are formed. There is a something akin to an aesthetic dimension to our unease. That is why infanticide is illegal and abortion is not. It is why, if the house is on fire, we save the baby in its cot before the frozen embryo. But this sense goes beyond what is visually familiar. Our moral imagination - the ability to conjure what lies beyond our direct experience - comes into play.

This is why the plan to abort one of a pair of twins has such resonance. The possibility of the psychological scars the experience may leave on the survivor is disturbing. But the utter arbitrariness of choosing one to live and the other to die hits home at this aesthetic disequilibrium, as much as at moral disquiet.

In seeing one of her developing babies as a child to be embraced and the other as comparatively disposable, this unhappy woman is doing more than wrestling with a personal dilemma. She is embodying the moral and aesthetic ambivalence of our whole society.