"I've asked my partner Adele if she will do me the honour of becoming my widow." With this announcement of his terminal cancer, Iain Banks has written one of the most memorable sentences of his lifetime, not just his life. I don't want to praise him as a writer, which would be presumptuous, nor do I have any special sorrow to express, except a fan's. What interests me right now is the light that his announcement casts on the meaning of marriage.

His aphorism has a weight and a symbolism that squashes a lot of the rhetoric about the meaning of marriage on both sides of the religious/secular debate. I have no reason to suppose that either Banks or his partner are any sort of Christian, but even if they were, what they are doing is clearly an expression of love that has nothing whatsoever to do with raising children. However, even Lord Carey and Keith O'Brien might hesitate before denouncing it on those grounds.

At the same time, it's not a private commitment, or something that only affects the couple in question. Such an idea of marriage rests on a kind of autonomy that just doesn't exist. Our relations with one person spill over into and affect our relations with others. Marriage is in important ways inaccessible to outsiders, but it isn't invisible to them. It couldn't be. So it will always be something that society has an interest in, and a perfectly legitimate one.

But society, in this instance, is no longer the church, and possibly never was, even in Scotland under Calvinist rule. Nor is it at all obvious that society here is the state. The state probably has to be concerned with marriage a bit, if only for tax purposes and legal tidiness. But I don't think you can argue that state recognition on its own constitutes a marriage, if only because marriages also end but some states don't recognise this and won't allow divorce. Clearly they are then wrong about who is actually married to whom.

So is marriage no more than a private contractual agreement between the spouses, which the rest of the world should take note of but otherwise ignore, like the mortgage I signed with my bank?

I don't think that quite works either. Marriage starts with a wedding, which is by definition a ceremony or a ritual. These need not be religious, though they have an almost overpowering tendency to appeal to some higher or greater power than just our own preferences. They have to, because they are an appeal to the deep currents of our nature, which are longer lasting and more powerful than the changeable weather of our feelings.

None of this need have anything to do with God, of course, but it is one of the foundations from which religious sentiments and practices are built.

The point about symbolic actions, like weddings, is that they are more fruitful and more powerful than symbolic noises, like words. We can't say them better than we can do them. Even so good a writer as Iain Banks is sometimes less eloquent in words. But words at least will preserve the memory of his action, and its power to offer meaning, long after his mouth is stopped with earth.