WE WILL ADMIT to a flicker or two of hope in the 15th round Wednesday night. There was a sudden flash of the old brilliance - the quickness, power and cunning that had twice gained for Muhammed Ali the heavyweight championship of the world. But it was gone as quickly as it had come. Youth reasserted itself, and Ali was just another beaten and battered fighter hanging in until the final bell.

Like most professional athletes, Ali could not bring himself to quit when his reflexes began to slow and his stamina to dwindle. He still may not be willing to bow out, given the dollars that roll in when he fights.

But he went to the well once too often, as they say, and Leon Spinks, a young and inexperienced boxer who had idolized Ali in earlier years, was the beneficiary. It was a disservice to both men that the officials were not unanimous in their verdict. Ali, we suspect, would be the last boxer to welcome the kind of sentimentality that made it a split decision.

Professional boxing is hardly our favorite sport. But we concede that Ali brought to it something it has almost always lacked. He has a special kind of class. Intelligent, witty, arrogant, a fellow of remarkable physical attributes, with an acute sense of theater - all this helped win him international fame and helped give boxing something approaching a good name. Ali, at his prime, may well have been the finest athlete of the last half-century. He certainly was one of the most canny. But no 36-year-old physique can stand off indefinitely the wear and tear inflicted on those who fight for a living in a ring. Never mind the nature of professional boxing; it is sad to see that the butterfly has slowed down - and the bee has lost his sting.