That adverb was in fact something of a misnomer. By "potentially" Blackmun and Western civilisation with him didn't really mean "potentiality" in the strict philosophical sense. Less developed foetuses are potentially separable from their mothers simply by virtue of the fact they are human and that's what humans tend to become. No, "viability" designated instead an actual here-and-now capacity for independent existence, by which of course was meant birth. These entities still deserving of defence could be born now. They could survive the onset of breathing and oral feeding — that's what the word "viability" encapsulated and what, since that time, has never really been contested. The only question has been when that moment comes — to which the answer depends in part on the state of medical technology. 

It was no coincidence that our culture chose viability as the pivotal point. There are other contenders: emerging biological characteristics such as the primitive streak; the detection of a heartbeat or brainwaves; the onset of foetal movement (the "quickening" so important to medieval thinkers); even the emergence of self-consciousness. But in an individualistic culture that ever since Rousseau has prized autonomy and detested dependency — "each of us, unable to dispense with the help of others, becomes so far weak and wretched", the fanatical Frenchman wrote in Emile, his manual on education — it is no surprise that we took viability, the first shoots of autonomy, as the all-important cut-off point. 

But step back a second. Because what are we really talking about here? What is the phenomenon in question? The natural reality in view? It is that human beings first appear in a state of radical dependency. We do not arrive in the world like the Greek gods — fully formed, instantly recognisable, immediately adult. We are not sown from the dragon's teeth, as in the autochthonous myth of the founding of Thebes, springing up as autonomous individuals. 

On the contrary: the animal that is to be king of the jungle begins as a weakling prince. Without exception we all appear in the world in the same way — in situations of total dependence, in the context of wholly asymmetrical relationships with our maternal hosts. And so the reality any discussion of morality, any wrestling with right and wrong, must take into account is what the German-Jewish philosopher Hans Jonas called "the radical insufficiency of the begotten". Nor is there anything pathological about our weakness, the fragility which characterises our earliest stages of development. Nothing has gone wrong to make this our way of appearing in the world. 

Yet if this is the way human beings come forward, what sense did it ever make to elevate viability into the ultimate criterion for entry into the community of people who, in the eyes of the law and in the standing of society, matter?