Most of those who voted for the 1967 Abortion Act did so in the clear belief that they were making provision for extreme and tragic situations – conception as a result of rape, foetal or perinatal complications threatening a mother's life and so on. Forty years on, a good many of these same people have expressed their dismay at what has in fact happened over this period. As we review these years - and as some of the issues are reopened in connection with the proposed legislation on embryo research – it may be important to think about where this unease comes from and whether it has any lessons for us now.

What they are saying, in effect, is that legislation intended to deal with exceptions, intending to tackle tragic circumstances that offered no simple right answer, has in its outworking had the effect of shifting some basic assumptions. Many of the supporters of the 67 Bill started from a strong sense of taking for granted the wrongness of ending an unborn life – and the same was undoubtedly true of those who campaigned for changes in the law in other countries. What people might now call their 'default position' was still that abortion was a profoundly undesirable thing and that a universal presumption of care for the foetus from the first moment of conception was the norm.

But the rapidly spiralling statistics – nearly 200,000 abortions a year in England and Wales – tell their own story. We are not now dealing with a relatively small number of extreme cases (and clinical advances have in fact reduced the number of strictly medical dilemmas envisaged by the 67 Act's supporters). When we hear, as a recent survey reported in the Lancet, that one third of pregnancies in Europe end in abortion, we may well ask – even granted that these figures are rather skewed by very high levels in the old Eastern bloc – what has happened. Recent discussion on making it simpler for women to administer abortion-inducing drugs at home underlines the growing belief that abortion is essentially a matter of individual decision and not the kind of major moral choice that should involve a sharing of perspective and judgement. And that necessarily means that certain presumptions have changed. Not only has there been an obvious weakening of the feeling that abortion is a last resort in cases of extreme danger or distress; the development of embryo research, whether we regard it positively or negatively, has brought with it the hint of a more instrumental approach to the human organism in its earliest days, which inevitably feeds in to the discussion of abortion – as is clear from the abortion-related amendments being debated in the context of proposed new legislation on this research.

Paradoxically, the language of 'foetal rights' has strengthened over the last few decades, leading to a real tension with this growing normalisation of abortion. The pregnant woman who smokes or drinks heavily is widely regarded as guilty of infringing the rights of her unborn child; yet at the same time, with no apparent sense of incongruity, there is discussion of the possibility of the liberty of the pregnant woman herself to perform the actions that will terminate a pregnancy. At the very least we need some joining-up here, even if it's only in the recognition that the model of competing rights or liberties (the mother's and the unborn child's) is not the most useful vehicle for a coherent moral grasp of the question.

None of this provides a knockdown argument for tightening the law or lowering the time threshold for abortions – though this latter issue needs attention if only because of the fact that the existing law assumes a rather less developed state of medical science than is now the case. The changes made in 1990 to the legal upper time-limit for abortion (from 28 to 24 weeks) reflect the need to keep this matter under regular review. But thinking about the processes by which we unconsciously shift what we take for granted does highlight questions about how we hold a steady moral focus in these matters of social and legal debate, questions which might take us beyond the current trading of slogans on the abortion issue.

We begin with clear, perhaps absolute, principles and, as we honestly confront a hugely complex world, we recognise that clear principles don't let you off the hook. There is no escaping the tough decisions where no answer will feel completely right and no option is without cost. But when do we get to the point where accepting the inevitability of tough decisions that may hurt the conscience has become so routine that we stop noticing that there ever was a strain on the conscience, let alone why that strain should be there at all?

The process is one that can be traced in other, perhaps more familiar, areas as well as the abortion question. You start with the presumption that abortion is unavoidably an act of violence, the ending of a life – but perhaps there are situations where it is the least awful outcome, and so you reluctantly conclude that some provision should be made for these situations. Or: you start from the presumption that marriage is a lifelong union – but it is appallingly cruel to refuse relief to people who are being systematically damaged by deeply unhappy marriages and you accept more rapid paths towards divorce. You take it for granted that marriage and family life are foundational things in a properly nurturing and stable society – but many mature and responsible people choose to live in partnerships other than marriage and because no-one wants to see them suffer hardship or discrimination because of this, you accept the case for the benefits of civil partnership. And it's difficult to deny that because of all these understandable and generous reasons something has happened to our assumptions about marriage and family - just as, for equally understandable and generous reasons, something has happened to our assumptions about the life of the unborn child. And the ongoing debate about assisted dying raises similar questions, as many have pointed out.

The history of the 1967 Act's implementation is an object lesson in how slippage can occur between thinking compassionately or flexibly about extreme and exceptional cases and losing the sense of a normative position. I don't think we're yet at the point where such a sense has been entirely lost. Even if some of the language about foetal rights is uncertain and confused, it illustrates the half-articulate conviction that the unborn child does merit protection. And the furore around Channel 4's recent broadcast about abortion with its vivid images of the unborn shows that there remains an instinctive recognition of humanity in the foetus even at very early stages, a recognition which advocates of more accessible abortion know they have to work to overcome.

But the slippage is there. This is not an argument for unalterable prohibitions in law against abortion in every circumstance - or against divorce or civil partnerships; there is room for disagreement over appropriate legal provision in all these areas. But it is an argument for keeping our eyes open – far more than we have done – for the unintended consequences, the erosion of something once taken for granted which occurs when we do not keep in focus the fundamental convictions about humanity that inform not only our responses to crisis but our routine relationships with each other. Precisely because we don't bring these convictions to light all that often, they can shift or weaken without our noticing. It's not a good habit for societies to get into; this debate, and the history of what has happened in the wake of the 67 Act, should remind us of some of the potential cost of such a habit in other areas.

© Rowan Williams 2007