THE DECISION of Justice Potter Stewart to retire from the Supreme Court can be accepted only with regret. He is stepping down long before most observers of the court thought he would -- or should. His departure will create a gap in the court's jurisprudence that cannot be easily filled.

When Justice Stewart came to Washington more than 20 years ago, his response to the inevitable question -- are you a liberal or a conservative? -- was, "I am a lawyer." The answer was just a small part of his larger determination not to be stereotyped. His sucess in that effort, and his record as a pragmatic, careful justice, can be judged by the term now most often applied to him: the "swing" man, a part of the court's middle.

Arriving at a time when the court was sharply divided on many issues, Justice Stewart quickly carved out his own place. Sometimes he was with the court's so-called "activists," led in those days by Justice Black, and sometimes with the advocates of judicial restraint, Justices Frankfurther and Harlan. More often, on critical cases, his vote was somewhere between as he slowly worked out an independent view of the Constitution and the law. In more recent years, changes in the court's personnel shifted his rather constant position somewhat to the left, to use a word he would reject out of hand as an appropriate description of any judicial opinion.

In addition to the loss of this important and influential voice in its deliverations, the court, the court loses two other things with Justice Stewart's retirement. His opinions were often marked with a literary grace that is all too uncommon in the law books; he loves to turn a felicitous phrase -- "enforced Sunday togetherness" was his description of the Sunday blue laws, and "yi know it when I see it" was the conclusion to his brief comment on the difficulty of defining obscenity. Justice Stewart's other unusual contribution to the court's work was his willingness to try to explain to laymen what the justices do. He talked more, and with more candor, in public than almost any justice in the court's history, but he almost never slipped into inappropriate subjects.

How does a president go about replacing a man like this on the nation's highest court? It is a difficult task and, given the likelihood that his is only the first of several Supreme Court noinations President Reagan will make, an extremely important one; the justices a president selects stay in power long after the president is gone. Mr. Reagan would do well to search the country for someone whose qualifications are much like those Potter Stewart brought with him from Ohio 22 years ago -- independence, thoughtfulness, a deep concern for striking a proper balance between the prerogatives of government and the freedom of individual citizens, and an ability to laugh at the absurdities of the law. The court needs such a voice at its center.