It's not easy to say what makes a definition right or wrong. There's a descriptive facet to it; for instance, if a dictionary were to define "apple" to mean "the fifth letter in the alphabet", they'd just be wrong because they'd be unfaithful to how that word is actually used. On the other hand, there's a prescriptive facet to the task: dictionary editors don't write their entries based on a simple poll of speakers. It could be the case, after all, that many people are just wrong about what a word means.

But when politicians are debating about "what 'marriage' means" I don't think these fine points about semantics and what goes into a correct definition are usually uppermost in their minds. The topic of "what 'marriage' means" is usually a euphemism for the question of what legal rights, economic advantages, and degree of approbation our society is prepared to bestow on gay couples.

Alex is right, I think, that people are not really debating what the word "marriage" means, though of course some politicians have been inclined to bring out their dictionaries. What's at issue is, rather, what the institution of marriage is. It's like the difference between a debate about what the word "flower" means and a debate about what flowers are.

I think philosophers do have a contribution to make here. Work in philosophy of language and mind over the last few decades has made philosophers very skeptical about the power and importance of "definitions". There are many cases to which one can point in which something that was, at one time, taken to be definitive of some phenomenon or kind of thing is later taken not even to be true of it. It was, for example, once regarded as part of the "definition" of "mammal" that mammals give birth to live young. The discovery of the platypus upset that "definition", and so it is now not even regarded as true that all mammals give birth to live young.

Similarly, even if it is true that, throughout history, marriage has been regarded as a relationship into which only one man and one woman could enter, and even if that has, for a long time, been regarded as partially definitive of what marriage is, it simply doesn't follow that marriage is a relationship into which only one man and one woman can enter. It might be that, as we reflect more deeply on the nature of such relationships, and particularly when we do so in light of the sorts of changes in our understanding of men and women and their social roles that have occurred over the last several decades, we come to believe that it isn't, in fact, essential to such relationships that they be between one man and one woman, though perhaps one can see in retrospect why that view was commonly held.

As I understand it, such reasoning was partly behind the decision of the Supreme Judicial Court in Massachusetts. There was a time, and it was not very long ago, when the contract of marriage essentially gave a man ownership of his wife. A man could not be prosecuted for beating his wife any more than he could be prosecuted for beating his donkey; he could not be prosecuted for raping her (rape itself was so understood that it would have been regarded as impossible by definition for a man to rape his wife); women had essentially no rights as regarded property; and so on and so forth. When marriage is so understood, and when the rights and roles of the two parties are so clearly defined by gender, then it is easy enough to see why marriage must be between a man and a woman. How would you know who owned whom otherwise? But all of that has changed, and we would now regard it, in retrospect, as deeply unjust that women should have been so treated. That is to say, our understanding of the kind of relationship marriage is has changed. And so what the court asked itself was: After all of these changes, is there anything left in the laws governing marriage that depends upon the genders of the two parties? The answer at which they arrived was that there is not.

Of course, there were more questions to be asked and answered before they arrived at their decision. But I myself wonder whether part of people's anxiety about gay marriage isn't fueled by a version of the question: How do we know who owns whom? Perhaps we don't regard the man as owning the woman any longer, but it's hardly true that, as a social matter, except for perhaps a few small circles, marital roles are no longer clearly defined by gender.