A landmark event in public education begins tonight at 9 when PBS will open its six-week series, "Hard Choices."

Most of us know something of bioethics, the new frontier between medicine and morality, the ever-broadening problem of our individual and collective responsibility that goes with the vast and frightening powers given us by science. But so far it has all seemed somewhat abstract and theoretical.

This series, guided by the humane and intelligent Dr. Willard Gaylin, president of the Hastings Center: Institute of Society, Ethics and the Life Sciences, brings the issues down to specific cases and awesome direct questions.

For starters, the question implicit in all the others: What precisely is a "right"? Who bequeaths it? Where does it come from? Does it extend to nonexistent people? That is, to generations yet unborn or conceived?

As one philosopher asks, "Because something is all right to do, does that make it a right?"

The first hour examines our new ability to find our the sex of a fetus in time to abort it. This is particularly useful to parents who fear Downs syndrome or some other genetic aberration. So far so good. But what of the parents who simply decide that they want a boy after several girls, or vice versa? What of the couple who stand to lose a huge inheritance if they fail to produce a boy to carry on the family name?

And what about a future in which, Gaylin notes, it may soon be possible to predetermine a baby's sex merely by taking a pill? Will this promote machismo values and sexism, looking children into sex roles far more than they are today? For example, many people say they'd like a son first, then a daughter, so the daughter will have "someone to look up to."

Next Friday's program cuts even closer to the bone, for it takes a hard look at genetic screening, as in the testing of blacks for sickle cell anemia. Screening can be helpful in warning young couples that they might be carriers of some such genetic flaw. But make the screening mandatory, as was done in the mid-'60s in certain cases, and you are toying with social dynamite.

"Who is to decide who's normal and who isn't?" Gaylin asks.

Along the way, "Hard Choices" presents fascinating views of the processes involved, from the drawing of fluid from a pregnant woman's uterus to the reading of chromosomes like a picture puzzle. There is also a natural-birth scene, which seems to be obligatory these days in public television science programs.

The series delves into human research subjects, behavior control, the rights of the dying, and -- perhaps the most appalling question of all -- who will pay for all this? For that reminds us that sooner or later the debate will turn political, and we face the prospect of Congress trying to decide issues utterly beyond its demonstrated competence.

For instance: Suppose we develop a vastly expensive heart-lung support system and make it available under national health care. But there are only so many machines. Can you imagine Congress being able to set an age limit for its use?

As Gaylin points out, one of these days we are going to have to rethink our attitudes and expectations about health care. We may soon have to redefine the very word health. In short, we are going to have to think about things we have always taken for granted up to now.