I was invited to debate at the Oxford Union last week, speaking against the motion that "This house believes there is an unrestricted right to abortion". I had got as far as working out what more or less what I would say when I discovered that I would be speaking alongside Stephen Green of Christian Voice. I won't do that, so I didn't.

Instead, I'm putting up a version of the speech here, shorn of the preliminary jokes.

I don't accept either of the two big competing theories on offer here: that abortion is wrong because a foetus has a human soul, or that it is a right that should be unrestricted.

I also want to say that I feel uneasy at being a man making any arguments at all. I don't think that this is solely a matter for women to decide: if I did, I wouldn't have an opinion at all. But I do think it is one that women pay most of the price of, either way; and I've known and some cases loved women who have had abortions. I can't believe they are murderers.

To deal with the Catholic absurdity first, the claim that an embryo or a very early foetus, is fully human seems grotesque. About a third of all conceptions end in natural abortions. In the great majority of cases the woman is entirely unaware of the process, and the embryo, of course, even less aware. The claim that these are the deaths of human beings is not to be taken seriously. And if they are not deaths when natural, how could they be murder when induced?

But at the same time I can't believe that the only question which matters is the convenience of the parents, or the capacity of the foetus for independent existence. My text here is taken from John Harris, a philosopher at Manchester who says he has not got a religious bone in his body. Because of this, he argues that there is something absurd in supposing that the moral status of a baby changes during its passage down the birth canal. One can sharpen the argument by imagining a caesarean section. If the knife slips and the baby is cut, that is no less wrong if it still inside the womb. So whatever it would be wrong to do to a newborn baby would be wrong also to do to one that is ready to be born.

Something of great moral importance has changed in the nine months of development from one cell to a baby; but like most biological changes, it's fuzzy. There is a "before", a "during" and an "After" but we can't easily point to the boundaries.

Now I think that Harris's argument is entirely sufficient on its own to establish that there is no unrestricted right to abortion that would not also extend to infanticide. And there is, I repeat, nothing religious about his reasoning at all.

But I want to go a little further and perhaps deeper into the swamp, and ask what is it about the new-born infant that makes infanticide wrong.

Let's assume for the purposes of argument that infanticide is in fact wrong, though this has been disputed in many times and places. There are plenty of examples of societies, from ancient Greece to modern North India, which have seen unwanted babies, especially females, much as we see surplus puppies: cute, but ultimately disposable; and there are hardly any which have had strong inhibitions against destroying enemy children. Certainly our own civilisation doesn't: from the use of "strategic bombing" to the sanctions against Iraq, we have repeatedly used the suffering and death of children as a political lever.

Still, for the sake of argument, let's say that we are all agreed that infanticide is wrong, and not to be used as a tool of policy by anyone in peacetime. So what is it about the infant which makes it wrong to kill one? I'm looking for a non-religious answer here, as the rules of the debate demand.

I am not sure that I can come up with a wholly convincing reply except that it just is wrong, in the same kind of way that torture is simply wrong. But I am pretty certain there is something wrong with an answer which is often heard in these debates: the idea that the crucial quality that makes for a proper human is autonomy.

This comes up in the context of abortion when discussing time limits. It is felt – and argued – to be grotesque that a baby can be aborted who might survive given sufficiently intensive care. In that case, it is said to be capable of independent existence. But of course it isn't. This doesn't change much at birth, either.

Anyone who has ever handled a new-born baby knows that they are incapable of independent life. Even the Greeks, who thought it virtuous to expose all new-born infants on a mountainside and raise only those who survived, did not leave the babies for more than a couple of hours to survive by themselves and when they did the results were invariably disastrous, as the story of Oedipus shows.

This dependence doesn't materially change at birth. It just becomes wider and more general. Even in the womb, and even an embryo or foetus never depends only and entirely on the mother, since the mother also depends on other people to feed her and keep her alive. That kind of mutual interdependence is part of what being human means.

This interpretation runs against the whole current of reasoning about rights. Interdependence can't be described or defined by contracts because the whole point of a contract is that it has conditions. But getting away from rights talk is surely a good thing. And it does offer one kind of answer to John Harris's question about what changes in the moral status of a baby when it is born. It's nothing in the baby itself. There's no special privilege to breathing air. But once it has been born, it has a claim on more than care. It has a claim on love.