When I was in the seventh grade, our science teacher explained to us in his own direct, blackboard language that the sun was gradually getting colder and would one day be totally dark.

At the time I was much more worried about my overhead smash than about the universe. The world, after all, wasn't going to get much colder than a New England January during my own lifetime.

Still, I remember thinking about that for months. The facts impressed me. I think that for the first time, they made me question how to live within the knowledge of some ultimate limit. That was, in any case, about as deep a dip into philosophy as I made at 12 or 13.

Since then, I've been a confirmed mathphobe who barely passed the college science requirements, but I've had a strong respect for the effect of scientific knowledge on the way we think about our own everyday lives. Whether it's the Black Hole or the Big Bank, science seems to be a strong impetus to introspection.

That's why I suppose I found Edward O. Wilson's new book, "On Human Nature," so compelling. This third major work by the "bug man" of Harvard, the controversial sociobiologist, is also about our limits. Not about the cosmic ones, but about the smallest ones, the ones called genes.

Arguments "on human nature" have raged since long before Darwin, but perhaps the most controversial question now is the one addressed in this work: "Is human nature essentially the product of biology or of culture?"

This isn't just another chicken-and-egg intellectual game. It's also a hot political issue, as Wilson discovered when he published his last book. Then, this mild-mannered Mister Rogers of a man, who had spent most of his career studying ants, found himself picketed and called everything from a racist to a fascist.

Many people reasoned that if human behavior is, at root, conditioned by culture, than it's fairly flexible. Change society and you change people. But if our behavior is directed (or dictated) by some inalterable biological machinery set in motion at the dawn of time, there's less room for change and therefore less reason to push for it, Sociobiology became a political issue.

Of course, most of us know that our lives are biologically determined if only by the need for sleep and food and that they are culturally conditioned if only in terms of language, customs or religion. But the question is one of percentages: how much biology, how much society? As Wilson asks: "But what is the ultimate range of our potential?"

The author's belief, expressed very simply is this: "The genes hold culture on a leash. The leash is very long, but inevitably values will be constrained in accordance with their effects on the human-gene pool." The first commandment of nature, he has said, is to do what is genetically advantageous.

In this book, which is smaller than the last 700-page, five-pound tome and much more speculative, he tried to describe both the "leash" and the slack on that leash. Making some giant leap from descriptions of insect behavior to people and from anthropology to present society, he offers his speculation about all sorts of human behavior.

He states badly that aggression, sexual differences, religious urges, and even altruism are biologically based, products of natural selection. But unlike many other sociobiologists, he doesn't go on then to justify either war or male dominance as the "inevitable" outcome of our biological destiny.

Rather, Wilson seems to think that we've only seen a small portion of human behavior that's possible. The cultures that inhibit or enhance, direct or redirect, this genetic potential represent only a fraction of what is possible.

As a work of science, "On Human Nature" is interesting philosophy.

The "bug man" broad jumps from his study of insects to his conclusions about human nature without always convincing me that there is solid ground between.

But I think he has written in part to goad the dialogue between biologists and sociologists - people who speak different tongues even when they are talking about the same society.

Sociobiologists are just beginning to try to prove the gentic roots of the way we are. They deserve a lot of skepticism both in terms of science and politics.But I think that they are open to a new way of thinking about ourselves and this can free us, free our minds even while it threatens us with a sense of human limits as chilling as those of the sun.