Fred is dead, and philosophic thoughts fill all minds at the Will home. One mind is especially somber. On the eve of her fifth birthday, Victoria, special friend and confidante of Fred the goldfish, has seen death and has decided, after mature deliberation, that life is still good. The time will come when she will see that there is much to be said for mortality.

Fred was named by Victoria for Fred Lynn, the Baltimore Orioles centerfielder. (Victoria certainly has got her father's number.) Fred lived into the sere and yellow leaf and died of the most natural cause, old age. Neither cholesterol nor automobiles nor handguns played a part. Irrational eating and drinking, negligence about exercise and similar foolish behaviors kill only creatures who can think and who therefore stand at the pinnacle of creation.

Victoria noticed Fred's decline the night before he died. Her attention wandered to the tank beside her bed, even though her father was giving an especially stirring reading of "Bread and Jam for Frances," a cautionary tale about a young badger who has an incontinent appetite for bread and jam. Victoria noticed that Fred, usually a trencherman, was peckish. At 10:23 the next morning, he died.

Victoria, having glimpsed, for the first time, the skull beneath the skin (in this case, the scales) of life, needed a few words from Leon Kass, a University of Chicago philosopher and biologist and medical doctor. Kass has just published a collection of essays to which I shall refer again in December when I award it the citation as the year's best book. It is "Toward a More Natural Science: Biology and Human Affairs" (Free Press). The essay Victoria needed in her sorrow is "Mortality and Morality: The Virtues of Finitude."

Kass says that retarding senescence and preserving youthfulness is part of the scientifc project of controlling biological aging. These objectives are continuous with the aspirations of medicine -- longer life, better health. But suppose sensational success; imagine an indefinite extension of life. Consider, Kass says, what would be lost.

Could life without the limit of mortality be serious? Homer's immortals are beautiful and vigorous -- and shallow and frivolous. Their passions are only transiently engaged. They are spectators of the moral dramas of life. Those dramas are the monopoly of mortals. As Kass says, "Mortality makes life matter -- not only in the chemist's sense." This is in part because finitude -- the sense of not having world enough and time -- is a spur to achievement.

Kass argues that not only seriousness but some beauty, too, is related to impermanence. He refers not just to the beauty of a flower or sunset, but especially to the distinctively human beauty of good character, of virtue. Immortals, says Kass, cannot be noble. They cannot meet the challenge of transcending concern with mere survival; they cannot put their lives at risk. (This indicates why pacifism, far from being a form of idealism, is a renunciation of all ideals in favor of a mere material thing, biological existence.) "Immortality," say Kass, "is a kind of oblivion -- like death itself."

The case Kass makes for mortality does not make a virtue of necessity. Rather it says that the necessity of death is the mother of virtue. The human longing that is assuaged by love and addressed by religion -- a longing deriving from the sense of incompleteness -- cannot be cured by longevity, however protracted. It cannot be cured by "more of the same."

Far from bringing happiness, the obsessive pursuit of longevity distracts us from the soul's natural quest. It is a distraction from the duty to master the fine art of living well, which requires rising above concern for mere bodily continuance.

Biology teaches what moral philosophy concludes: We are social, communal creatures, with strong impulses, physical and spiritual, for reproduction. We are constituted for concern for the species; our lives point beyond themselves, toward perpetuation. Children are our participation in the enduring.

A craving for physical immortality is childish in the sense of being narcissistic and incompatible with a mature devotion to posterity. It also is hostile to children. Children are the bearers of our hopes, and if they are to flower, Kass notes, "we must wither and give ground."

Those who come after, who take our place, are "life's answer to mortality, and their presence in one's house is a constant reminder that one no longer belongs to the frontier generation." That is why to have children is to come as close as is possible to reconciliation with the human condition.

That is what Fred's death, and Victoria's tears, caused me, with Kass's incomparable help, to think. But all I told Victoria is that Fred is in fish heaven. She can read Kass when she has mastered "Frances."

Fred now rests beneath Maryland soil. His tank is home to a member of the next generation, Eddie. Eddie is named for Eddie Murray, the Orioles first baseman. Victoria, obviously, is fine.