Pat: Gay marriage is a mistake because marriage is about children. Two men or two women can’t have children together accidentally or on purpose.

Mike: So if you’re infertile, you can’t get married?

A lot of time gets wasted in arguments that are really about what the proper unit of analysis is, without any of the participants quite realizing that is what their argument is about.

Let’s take gay marriage, for example. Defenders of the traditional definition of marriage believe that marriage is fundamentally tied to procreation. Proponents of gay marriage pooh-pooh the suggestion. The defenders, they point out, do not try to prevent old or infertile couples from getting married, nor do they try to prevent couples who have decided not to have children from getting married.

The defenders reply that heterosexual couples that are trying not to have children can still accidentally have them, as can in rare cases old people, and that infertile couples also sometimes have children (the phenomenon of a couple trying various fertility treatments and finally giving up on having children in frustration, and then conceiving a child without meaning to, is common enough). Besides, the defenders add, even infertile or old heterosexual couples serve as a model to other heterosexual couples who can have children.

Then the debate descends into unknowable speculation about the extent to which gay couples can also serve as a model for heterosexual couples who care capable of having children.

But what they are really arguing about is the proper unit of analysis. What they are really arguing about is the proper level of abstraction.

The question arises, in deciding on public marriage policy, are we evaluating classes of relationships (gay, heterosexual) are we evaluating specific relationships (Pat and Mike, who even if heterosexual may still be infertile) or are we evaluating specific relationship acts (Pat and Mike in the parlor on Tuesday, which may be an infertile conjugal act for all sorts of reasons, even if Pat and Mike are capable of having children).

Without a very articulated reason why, the defenders are suggesting that the class is what should be evaluated. Without a very articulated reason why, the proponents are saying that the specific relationships are the proper unit of analysis.

Proponents aren’t very consistent, though. They see the purpose of marriage as being recognizing a committed, loving relationship. They argue that the state should recognize gay marriage because gay couples, as a class, are capable of a committed, loving relationship. But they don’t argue that couples who don’t appear to be sufficiently in love or who have a history of failed relationships should be barred from marriage. But far more married couples turn out not to have had a committed, loving relationship then turn out to have been infertile.