WITH his retirement, Thurgood Marshall takes away more than just one of the few remaining liberal votes on the Supreme Court. More, even, than his passionate written opinions - in recent years, most often in dissent. Justice Marshall takes from the Brethren's deliberations a living witness to the lives of the poor and powerless in America that is unique in the court's history.Marshall was not the first Supreme Court justice who rose out of humble beginnings. But he is the only justice in history who was denied admission to a law school because he is black. The only justice who was ever threatened with lynching. The only one who ever defended in court men and women whose only crime was the color of their skin, and whose rights were calibrated according to that same standard. Largely because of Marshall's work as the greatest civil rights lawyer of the 20th century, no future Supreme Court justice, not even a black one, will have experienced the effects of American segregation and discrimination as profoundly as he did. The absence of Jim Crow doesn't mean, however, that justices can safely be cut off from contact with the real-world conditions in which too many Americans live. Marshall never let his colleagues forget what the system looks like staring up from the bottom. If that perspective is no longer available to the justices from within the court, they will have to reach out conscientiously to bring it into their deliberations from other sources. Marshall won't sit on the All-Time Nine beside Holmes, Brandeis, Black, Warren. If in the future his dissenting views on civil rights, criminal procedure, or capital punishment become the law, it's likely that later majorities will look more to the opinions of William Brennan, who retired last year, than to Marshall's writings. But Marshall, as the chief architect of the successful fight against racial discrimination in the nation's courts, is a great American. With Marshall's replacement, the conservatives' 6-3 or 5-4 victories of recent terms in some areas of the law could become 7-2 routs. The conservative majority isn't monolithic, however; the reasoning behind many of its outcomes is fragmented. Marshall's successor can play an important role in determining the kind of conservative thinking that will dominate American jurisprudence for a generation. The next justice won't share all of Thurgood Marshall's views. But we hope President Bush will appoint a man or woman who shares Marshall's grandeur of spirit and his empathy for those struggling on the fringes of American life.