Abortion is now a non-issue because, in the public mind at least, the debate has been framed as a stand-off between religion and secular philosophy. While faith is thought to elicit a broadly pro-life position, reason supposedly supports a pro-choice one. But since faith rests on unverifiable claims — you can almost hear the mental cogs grinding — it can hardly provide a platform for policy, leaving us with an intellectually unassailable justification for abortion. 

The problem is, however, that when you click the "Accept" button confirming those terms and conditions, something important happens: you inadvertently smuggle in the assumption that the philosophy underpinning the pro-choice position was, in itself, coherent. That it was robust and truthful. That it made sense of the world as it is, not as we might have wanted it to be. That it had an essential purchase on reality. That it was timeless truth rather than a particular paradigm riding high at one particular moment in history-in short, a specific strain of liberal, Anglophone, late-20th-century moral and political philosophy.

But now there are many thinkers — most prominently, postmodern ones — who lead us to question how good that philosophy was. How good in terms of accounting for the human condition. How good at fitting the facts.

It is with the whole concept of "viability" that the philosophy really falls down. Forty-six years ago, when parliament passed the Abortion Act, it did not declare all pregnancy terminations legal. It didn't say that every creature resident in its mother's womb was now outside the protection of the law. Rather, it established what was effectively a two-tier response to abortion, with broad defences covering abortions carried out in the early part of pregnancy and a more restrictive response to those carried out after 28 (now amended to 24) weeks. Why 28 weeks? Because, crucially, that was when the foetus was thought to be "viable", described in an earlier piece of legislation, the Infant Life (Preservation) Act 1929, as the point at which the foetus was "capable of being born alive". This was the point that was picked, the point when the state accepts a compelling interest to safeguard human life, when we may rightly think of the new one as our equal (of sorts), as an entity that is to be afforded increased protection. 

Six years later, the American judiciary followed the British legislature in also selecting viability as the threshold below which termination was permitted. In the landmark case Roe v. Wade (1973) Justice Blackmun defined viability similarly: as the moment when a foetus becomes "potentially able to live outside the mother's womb, albeit with artificial aid".