THERE'S PROBABLY NO ONE literate above the age of four who doesn't know by now that detectives have arrested a man they'd believe to be "Son of Sam," the maimer and killer of young women and their boyfriends, who has been terrorizing New York City for more than a year. And there's surely no one with any sensibility at all who doesn't hope the police have got their man and feel some measure of relief - not just those who happen to live within the area Son of Sam had made his battleground, but also those who dwell comfortably outside his accustomed range of fire.

These sentiments are irrestible. They also tell us something about ourselves and about the crime-battered world we live in. The deranged mentality that staged, one after the next, the Son of Sam shootings was not in what you would call mainstream of urban American crime. The killer's pattern was aberrant, outside our realm of prediction and understanding, motivated - driven, even - but by what? Because he seemed to live in a world wholly alien to our assumptions and expectations, including our assumptions about how criminals behave, the killer called Son of Sam created more anxiety and fright than other gunmen who might inflicts as much death and damage on innocent victims. He was not, you might say, your ordinary, run-of-the-mill killer. He was, therefore, more frightening.

This strange circumstances illustrates how much we have come to accommodate run-of-the-mill crime to our daily existence, accepting a certain level of violence as the product of forces outside our immediate range of control or remedy. It also shows that the kind of aberrant crime Son of Sam embodied is more vulnerable to police action than is the pervasive, everyday kind. The painstaking, record-sifting operation set up by the police in New York, with all their relevance in trying to deal with the kinds of crime that terrorize parts of our cities day and night, month in, month out.

We join everyone else in the fervent hope that the New York police have arrested the right suspect. We think they were right to have channeled so much of the force into the effort to stop the maniacal Son of Sam killings. We wish them well on their return to the full-time pursuit of less exotic - and harder to remedy - violent crime.