The stories have been on my desk for weeks. They are haunting morality tales, about families and doctors, life and death.

On is the story of the Siamese twins inseparably joined at the waist with three legs. It tells how the parents, an Illinois couple, were prosecuted on charges (later dropped) of withholding treatment and food.

Another is a longer story about doctors who "saved" a severely brain-damaged infant. The parents did not want this child to live.

Below these stories a report, less emotional, less highly charged, of the presidential commission which suggested that we redefine death as the irreversible cessation of the entire brain. Behind this report are case histories of people currently kept "alive" on machines. Behind it also are protest letters from people who believe that such a law is a "stepping stone to . . . euthanasia."

What is one to say about these stories? That they represent human tragedies? That is obvious. That they are examples of some diminishing value of life in this society? That is not so obvious.

Something curious has happened to us. This highly technological society has come up against the same moral dilemma faced every day by the most primitive societies: Who shall live and who shall die and who shall decide?

These are questions we thought we might avoid, through science, through prosperity. Instead we are asking them at different but equally troubling levels.

Societies have always gauged their membership by what they could afford. The shallow graves that dot our human history are the bleakest testimony to that fact. They were not the work of unfeeling monsters but of desperate parents.

Our ancestors, I imagine, were seared by the necessity of making the most painful moral decisions. If five could barely subsist on the food available and a sixth meant doom for all, they had to ask whether it was moral, even sane, to let the sixth live. Nor was it simple callousness that permitted those in some harsh primitive cultures to expel their aged, weak or ailing parents. Even the cruelest adult would not have predestined his own end if he had a choice.

The truth is that every society, whether a family, a tribe, or a nation, makes moral decisions about life. How much of the energy of a society can be, should be, devoted to the weakest members? What is right and what is wrong . . . and yes, what can it afford?

Similar decisions are made now, here, over abortion. But also about the born. The very technology that has made it possible to feed the infants we give birth to, to sustain more of the sick and old, has presented us with a sophisticated version of the most basic ethical dilemma.

The ability to "save" many -- a Siamese twin, an infant with spina bifida, another with severe brain damage, a third with complications accompanying Down's syndrome -- allows and forces families to choose what was once fate. It allows and forces families to ask whether, and when, we should do what we can do.

The same technical ability, this time to keep someone "alive" one a heart-lung machine after the brain has stopped functioning, allows and forces us to define what life is.

It would be easiest to write out a single "defense of life" in any form, to save every infant, every breathing being, regardless of how it will live and what its effect will be on the life of others.

We could even pretend that we have unlimited resources -- emotional, financial, physical -- to spend. It is scary, after all, to decide on the value of a "subnormal" life, to draw lines around a life worth living.

But the costs that spiral with out technological aids can overwhelm our society as much as any primitive one. For all the pains involved, the moral and ethical questions we face are necessary and legitimate ones.

We, too, make choices about how we allocate our limited financial -- among those who need them. How much should be, can be, used to maintain human beings who cannot think or respond, but only breathe? How much of a family's energy and life can be, should be, drained by those who barely exist?

There are no blanket answers to these matters, and so we try, individually, case by case, to sort out the right and wrong. And the pile of cases on my desk grows higher and higher.