How can you decide whether to have a child? It's a complex and profound question -- a philosophical question. But it's not a question traditional philosophers thought about much. In fact, the index of the 1967 'Encyclopedia of Philosophy' had only four references to children at all -- though there were hundreds of references to angels. You could read our deepest thinkers and conclude that humans reproduced through asexual cloning.

Recently, though, the distinguishedphilosopher L.A. Paul (who usually works on abstruse problems in the metaphysics of causation) wrote a fascinating paper, forthcoming in the journal Res Philosophica. Prof. Paul argues that there is no rational way to decide to have children -- or not to have them.

How do we make a rational decision? The classic answer is that we imagine the outcomes of different courses of action. Then we consider both the value and the probability of each outcome. Finally, we choose the option with the highest 'utilities,' as the economists say. Does the glow of a baby's smile outweigh all those sleepless nights?

It's not just economists. You can find the same picture in the advice columns of Vogue and Parenting. Today, we assume that we can decide whether to have children based on what we think the experience of having a child will be like.

But Prof. Paul thinks there's a catch. The trouble is that, notoriously, there is no way to really know what having a child is like until you actually have one. You might get hints from watching other people's children. But that overwhelming feeling of love for this one particular baby just isn't something you can understand beforehand. You may not even like other kids much and yet discover that you love your own child more than anything. Of course, you also can't really understand the crushing responsibility beforehand, either. So, Prof. Paul says, you just can't make the decision rationally.

I think the problem may be even worse. Rational decision-making assumes there is a single person with the same values before and after the decision. If I'm trying to decide whether to buy peaches or pears, I can safely assume that if I prefer peaches now, the same 'I' will prefer them after my purchase. But what if making the decision turns me into a different person with different values?

Part of what makes having a child such a morally transformative experience is the fact that my child's well-being can genuinely be more important to me than my own. It may sound melodramatic to say that I would give my life for my children, but, of course, that's exactly what every parent does all the time, in ways both large and small.

Once I commit myself to a child, I'm literally not the same person I was before. My ego has expanded to include another person even though -- especially though -- that person is utterly helpless and unable to reciprocate.

The person I am before I have children has to make a decision for the person I will be afterward. If I have kids, chances are that my future self will care more about them than just about anything else, even her own happiness, and she'll be unable to imagine life without them. But, of course, if I don't have kids, my future self will also be a different person, with different interests and values. Deciding whether to have children isn't just a matter of deciding what you want. It means deciding who you're going to be.