Our text leaves most of this out, but for the sake of getting a fuller picture of Marquis' argument, I include it in our notes. Framed in its usual terms, Marquis believes that the abortion debate results in a stand-off between the two sides. In section I, he attempts to demonstrate this by reviewing certain typical features of the debate. Then they conclude that abortion is not acceptable. But as Marquis points out, neither side can rest on such facts alone.
Moral conclusions require moral premises.
And indeed, each side can add plausible sounding premises. Here, however, each side seems to miss the mark. The pro-life principle is too broad. It might well show that killing a cancer-cell culture is immoral, or so Marquis claims. On the other hand, the pro-choice is too narrow.