I'd like to say a word in defense of conventions. It's something I've been thinking about since a recent conversation with a friend.

My friend's 20-ish daughter, who lives at home, has a boyfriend who has his own apartment. My friend is trying to brace herself to deal with the inevitable day when daughter decides to sleep over at boyfriend's house.

"I could ask her not to do it just because it would make me very uncomfortable (although I'm not sure how good a reason that is), or I could order her not to do it," she said. "Either approach would probably make her all the more determined to do it.

"It really would make me very uncomfortable, although I'm not sure how valid my reasons are, or how well I could defend them. It isn't a question of morality, or protecting her from anything.

"In the first place, she is a decent girl. In the second place, I know that anything she can do at 5 a.m., she could do just as easily at 5 p.m. I'm not even sure I object all that strongly to her having sex, since she and the boy have been dating seriously for a long time now.

"Am I just being hypocritical?"

She was afraid that if she were required to give a point-by-point argument in favor of her position, she might wind up looking silly. I told her that it's a trap even to try to justify moral positions on the basis of logic. There's no law that says we have to think everything through.

Aren't we better off if we simply behave conventionally when it comes to these routine, recurring questions of behavior and save our thinking-through for those occasions when the conventions don't serve our needs?

I mean, you didn't think through the question of whether you would put on clothes before you went out this morning, although you clearly could have.

You might have considered the weather and your likelihood of catching cold. You could have thought about personal comfort and the time you could save by not getting dressed, and you might have weighed all that against the probabiltiy that your public nudity would give others an unfavorable impression of you, or that you might get arrested or carted off to the funny farm. And you might have ended up deciding that, on balance, it was better to get dressed.

Or you might simply have accepted the conventin that forbids public nakedness. And unless there was some overriding reason to defy the convention (the house being on fire, for instance), you would do the conventional thing.

I think of conventions (I told my firiend) in much the same way I think of traffic lights. I stop for the red lights, even if it's 3 a.m. and obvious that the way is clear. But I don't wait forever if it becomes apparent that the light is stuck. If I'm delivering a gravely ill family member to the hospital emergency room, I might check for cross-traffic and go through the red light.

The point is that under routine circumstances there is no need to think the matter through. The convention serves well enough.

It may happen that some streets are routinely so traffic-free in the wee hours that "taking" red lights becomes the norm. And if that happens, traffic engineers might decide to replace the regular signal with a flashing one.

In other words, when the conventions no longer seem reasonalbe to large numbers of people, the conventions change. But no sensible traffic engineer would remove all the controls, leaving it to each individual driver to decide when to proceed. That is an invitation to chaos, and so is the present tendency to look at conventions as impediments to rational behavior. Conventions are the moral equivalent of morality.

They save the mental wear of thinking through every single proposition, whether it involves nudity, sleeping over or incest. The fact that every one of these conventions is breached now and again -- that there is always someone among us who will decide to cross the line -- does not diminish the importance of having clearly established lines.

Conventions aren't so much illogical as alogical -- even supralogical. They represent the conclusions of the whole society and, as such, serve to simplify our lives.

My friend finally decided to fall back on the oldest of conventions: nice girls don't do that. Or at least nice girls don't do that and stay in this house.

I'm nor aware that her daughter is suffering unduly.