Full Idea
The case against abortion rests either on the claim that the foetus is a human being (or a potential human being), or on the different claim that the foetus is a person (or potential person).
Gist of Idea
If abortion is wrong, it is because a foetus is a human being or a person (or potentially so)
Source
Jonathan Glover (Causing Death and Saving Lives [1977], §9)
Book Reference
Glover,Jonathan: 'Causing Death and Saving Lives' [Penguin 1982], p.120
A Reaction
The obvious problem with 'potential' is that every time Jack meets Jill there is a potential birth. And an early foetus is barely human, and clearly not a person.