James N. Anderson writes,
To grant that marriage could be redefined is to capitulate to a postmodernist anti-realism according to which all social structures and institutions are mere human conventions and there is really no such thing as human nature, understood in traditional metaphysical terms. We must insist that marriage is not something that can be defined and redefined as we see fit. Marriage is a divine institution, not a human social construction like chess or money that we invented for our own purposes. There wasn’t a point in time at which humans ‘defined’ marriage in the way that, say, a foot was once defined as 12 inches. Marriage was bestowed upon us, not created by us.
1. It is certainly true that if marriage is a divine institution, as Professor Anderson says, then it has a nature not subject to human definition or redefinition. For if there are natures, then they are what they are whatever we say about them or think about them. They are what they are whether we frame definitions of them or fail to do so, or do so accurately or inaccurately. But that marriage is a divine institution is a premise that won't be granted by many and perhaps most of the participants in the current debate over same-sex marriage. It is therefore futile to use this premise in the current debate. Or as the pugnacious Irishman Bill O'Reilly said the other night, "No Bible-thumping." Defenders of marriage ought to invoke only those premises that secularists could accept, assuming that the goal is either to persuade them that the traditonal concept of marriage ought not be revised, or to show them that traditionalists have a principled stand that does not arise from biogotry or a desire to discriminate unjustly.
Suppose I want to convince you of something. I must use premises that you accept. For if I mount an argument sporting one or more premises that you do not accept, you will point to that premise or those premises and pronounce my argument unsound no matter how rigorous and cogent my reasoning. I am not saying that marriage is not divinely ordained; I am saying that the claim that it is has no place in a discussion in which the goal is to work out an agreement that will be acceptable to a large group of people, including theists and atheists. (Not that I am sanguine that any such agreement is in the offing.)
2. Whether or not marriage is a divine institution, it can have a nature. That is: the question whether marriage has a nature, and the question whether there are natures at all, are logically independent of the question whether God is the ultimate ontological ground of natures. Or at least this is prima facie the case. Jean-Paul Sartre famously maintained that man cannot have a nature because there is no God to give him one; but it is not at all clear that a godless universe must be one bereft of natures. Aristotle believed in natures even though his Prime Mover was neither the creator nor the ontological ground of natures.
3. Let's assume that there is no God, and that therefore marriage is not divinely instituted, but that some things have natures and some things do not. Water, to coin an example, has a nature, and it took natural philosophers a long time to figure out what it is. Chess, by contrast, does not have a nature. It is a tissue of conventions, an invention of man.
4. Does marriage have a nature? If it has a nature, and that nature requires that marriage be between exactly one man and exactly one woman, then there can be no question of redefining 'marriage' so as to include same-sex 'marriages.' If marriage has the nature just specified, then it is impossible that there be such a thing as same-sex marriage. And if same-sex 'marriage' is impossible, then one cannot sensibly be for it or against it. 'I am for same-sex marriage' would then be on a par with 'I am for carnivorous rabbits.'
'Should homosexuals be allowed to marry?' for traditionalists is like 'Should cats be allowed to philosophize?' The nature of cats is such as to rule out their doing any such thing. Similary, on the traditionalist understanding, marriage has a nature, and its nature is such as to rule out tlhe very possibility of same-sex 'marriage.'
5. Any talk of redefining 'marriage' therefore begs the crucial question as to whether or not marriage has a nature. Such talk presupposes that it does not.
6. If the same-sexer goes POMO on us and adopts antirealism across the board, then he opens himself up to a crapstorm of powerful objections. But needn't go that route. If Anderson is suggesting that the same-sexer must, then I disagree with him. The same-sexer need not embrace antirealism along the lines of a Goodmaniacal worldmaking constructivism; he might simply claim that while there are natures, and some things have them, marriage is not one of those things.
7. Can I show that marriage has a nature? Well, there is very little that one can SHOW in philosophy, so let's retreat a bit. Can I make a plausible case that marriage has a nature? Well, man has a nature and certain powers grounded in that nature, one of them being the power to procreate. The powers of human beings are not like the 'powers' of the chess pieces. It is by arbitrary human stipulation that the bishops move along diagonals only, capture in the same way they move, etc. But the power of a man and woman to produce offspring is not a power that derives from arbitrary human stipulation. It is a a power grounded in the nature of human beings.
Now if 'marriage' refers to what has traditionally been called marriage, i.e., to that the definition of which the same-sexer revisionists want to revise so as to include same-sex unions, then 'marriage' refers to a relation between opposite-sexed human animals that is oriented toward procreation. Of course there are social and cultural factors in addition to this natural substratum. There is more to human marriage than animal mating and care of offspring. But if you grant that human beings have a nature and a procreative power grounded in this nature, then it seems you have to grant that 'marriage' refers to a union between opposite-sexed human beings, a union that has a specific nature. If so, then it is senseless to want to revise the definition of 'marriage.' Marriage is what it is; it has a nature, and that's the end of it.
UPDATE (4/26): Professor Anderson responds here.