This extract is taken from a longer discussion with Professor Jim Stone on Jonathan Glover's website.

Consider the second premise, that there are no morally relevant differences between infants and fetuses. This is used to extend the protection we give to infants all the way back to conception. The first thing that should make us uneasy about this strategy is that it works just as well in the other direction. To many people, their belief that taking the morning-after pill is not murdering someone has as much intuitive force as the belief that infants have a right to life. If infants and newly fertilized eggs do not differ in morally relevant ways, accepting either of these beliefs commits us to denying the other. We can slide smoothly from platitude to paradox either in one direction with Professor Stone, or in the other direction with some of his opponents. Perhaps the premise which allows this needs more careful scrutiny.

The apparently simple premise, that there are no morally relevant differences between infants and fetuses, blurs an important distinction between what can be called external and internal differences. Someone's external characteristics are constituted by their relationships with others, while internal characteristics are independent of those relationships. The distinction is not always sharp, but an example may make it clear. Suppose there is a fire, and I can save some, but not all, of those threatened. If I save my own children in preference to other children, some very severe equal rights theorist might think I have shown unjustifiable discrimination. If only internal features are relevant, other children had an equal claim. But if external features count, the fact that they are my children may be morally relevant. The plausible-seeming claim that infants and fetuses do not differ in morally relevant ways depends on the difficulty of citing relevant internal differences. It assumes that our relationship with an infant is irrelevant. This assumption should be questioned. We also need not accept that a newly fertilized egg is morally of the same standing as a late fetus. Where differences are of degree, difficulty in justifying sharp lines proves less than is often supposed. There is no sharp line between some leaves and a heap of leaves, but a hundred leaves piled on top of each other are a heap and one leaf is not.