In a small town on Long Island, a 16-year-old girl pleads guilty to having paid a classmate $400 to kill her father because, she claims, he had been abusing her sexually and she now feared that he was about to start on her younger sister as well.

To make this horrible story still more horrible, the murdered man's mother believes that her granddaughter is lying about having been forced to sleep with him. She says that the girl had other motives for the murder.

Unlike her grandmother, the prosecutor in the case is willing to stipulate that the girl is telling the truth. Nevertheless, in arguing against too lenient a sentence, he takes the position that she was still not justified in having her father murdered. There were, he maintains, other courses she could have followed in trying to deal with her problem.

Just after reading about this case, I happened to see a new movie called ''The Rosary Murders'' in which a girl's response to being forced into sexual relations with her father is to kill not him but herself. The father, a Catholic, then sets out to murder all the priests and nuns who had failed to do anything either for him or his daughter when they appealed for help.

It is an absurd plot for a murder mystery. Yet when at the end the guilty father bitterly declares that all he had been given after confessing his incest to a priest was a penance of ''five Hail Marys'' and an admonition to stop, I found my thoughts returning to the prosecutor's argument in the Long Island case.

No doubt this girl could have gone to the authorities. But what would they have done? Would they have pressed criminal charges against her father?

Possibly. Chances are, though, that they would have shared the view I heard expressed on television the other day by a panel of social workers discussing pedophilia, which is the technical term for lusting after children. These social workers had not yet reached the point of condoning, let alone advocating, pedophilia. But neither did they manifest any special horror at the thought of it. To them the compulsion to seduce or rape children was just another illness, comparable to alcoholism.

When applied to pedophiles whose lust is directed at their own children and who then submit to it and commit incest, this line of reasoning leads straight to ''family therapy'' -- the secular equivalent of the ''five Hail Marys'' and the admonition in the film ''The Rosary Murders.''

In such a therapeutic setting, the ''problem'' is brought out into the open; it is ''aired'' and ''explored in depth.'' As a result the guilty father presumably learns to control himself, while the abused daughter and the rest of the family all come to understand and forgive one another and then to live happily ever after. Well, maybe not so happily, but who after all is happy?

In this attitude toward incest we see the ultimate ''triumph of the therapeutic,'' as the sociologist Philip Rieff once called it, over the moral view of life.

It is the ultimate triumph because, up until practically yesterday, pedophilia in general and incest in particular inspired an instinctive and reflexive horror in practically everyone. Indeed, nothing in the realm of human abominations seemed more self-evidently evil to more people than the sexual molestation of a child, especially a child of one's own. Thus, in turning even this most elemental violation of the moral order into an illness, we have finally hit the bottom of a slippery slope down which our society has been sliding for nearly a century now.

I am reminded here of a famous debate between two British literary critics back in the 1940s.

One of them, Lord David Cecil, had written patronizingly of the great Victorian novelist George Eliot as a Puritan who admired chastity and self-restraint, and disapproved of loose living and self-indulgence. To this patronizing essay the other critic, F. R. Leavis, replied sarcastically that, at the risk of exposing himself to universal ridicule in the literary world, he had to confess that he shared George Eliot's admirations and disapprovals. Moreover, he added, ''the enlightenment or aestheticism or sophistication that feels an amused superiority to them leads, in my view, to triviality and boredom, and . . . out of triviality comes evil.''

In America today, the superiority our own enlightened and sophisticated classes feel to the values Leavis was defending is not usually ''amused.'' It tends, rather, to be even more earnest and more solemn than the Victorian ideas it has so thoroughly routed and replaced. But it leads all the same to precisely the triviality and boredom Leavis was warning against. For what could be more trivial and more boring than a life in which no act, not even incest, matters enough to inspire horror and revulsion?

And Leavis was right too in saying that out of this very denial of evil comes more evil. As the unspeakable becomes a casual subject of babble and chatter, restraints and fears are shed that would otherwise have deterred. What once would have seemed inconceivable now becomes possible. What once would have seemed too weighty a burden for the spirit to bear now becomes fairly easy to live with.

The girl in Long Island who committed parricide -- a crime as elemental as incest in its assault on the moral order -- was at least doing something commensurate in seriousness with the evil that had been done unto her. Which is more than could have been expected from any action by the authorities to whom the prosecutor retroactively advises her to have gone instead.