Among the weaknesses incidental to humanity is a reluctance to credit eminent persons with commonplace motives. But it is both sensible and civil to note that Edward M. Kennedy is a conscientious father with many children -- his own and those of two brothers -- about whom to be conscientious.

The fact that his decision to take himself out of the 1984 presidential race has been made now suggests that personal rather than political considerations were paramount. To whatever extent political calculations about 1984 were involved, to that extent it made sense to wait and see if the economy, which is on a knife-edge, turns down so drastically that the country becomes receptive to any candidate who is not a Republican. That is the condition required for a Kennedy candidacy to seem worthwhile.

Furthermore, some undertakings are so grindingly arduous that they cannot be done well except by persons who relish the draining strain. Being a surgeon is one; being a professional football lineman is another; being a presidential candidate is a third. During 1980 Kennedy became a better candidate, but as in so much of his life, he seemed cast in a role written by a destiny he vaguely regretted.

Kennedy is neither a masochist nor a fool. Indeed, he has a well-attested appetite for pleasure, and has political chromosomes. He knows better than any living American that campaigning for president is not fun and that, for him, it is not safe. Furthermore, he knows that the next time he loses will be his last loss in presidential competition.

He also knows how to read election returns, having been reading them since his brother, John, ran for Congress in 1946, when he, Ted, was 14. It is all very well to remember Kennedy's skillful sermon that so pleased the choir in Madison Square Garden 28 months ago. But by then he had been trounced by a Democratic opponent incapable of kindling Democrats' passions -- an opponent who then lost 44 states.

The 1982 election returns could not have been encouraging to Kennedy. Consider the gubernatorial races in two of the states a Democratic presidential nominee must carry.

In New York, the Democratic candidate, Mario Cuomo, an intelligent, traditional Democrat, won. But he won only narrowly against a Republican (Lewis Lehrman) who scandalized the Republican establishment by suggesting that Ronald Reagan's Reaganism is tepid, and promised the real thing. In Michigan, the Democratic candidate, James Blanchard, won, but only narrowly in a strong labor state that is in the throes of a depression. His opponent, Richard Hedlee, was opposed by much of the Republican establishment because he, too, drinks Reaganism the way Scots drink Scotch -- warm and neat.

This does not mean that the country is "moving right." It does mean that the sands are shifting beneath the parties' feet in ways that are unpredictable but not encouraging to Democrats counting on a pendular swing back toward the political patterns that have benefited Democrats throughout the postwar era.

Fish gotta swim, birds gotta fly and Washington gotta wonder what this means for other Democratic candidates, who will now rise from the underbrush like rocketing pheasants. Perhaps it helps Walter Mondale, who has consistently risen passively, as a result of the actions of others. (He was appointed attorney general of Minnesota in 1960 when the incumbent resigned. He was appointed to the Senate in 1964 when Hubert Humphrey became vice president. He was plucked up as vice presidential candidate after an aborted presidential campaign.) Now Mondale is, by default, suddenly the front-runner. Concerning the joys of the role, he can consult the experiences of George Romney in 1968 and Ed Muskie in 1972.

Kennedy was in the incongruous position of being a young man but an "old face." Now the old face is Mondale. If Kennedy's withdrawal works as an invigorating tonic on Democrats, quickening their sense of adventure by enlarging their sense of possibility, then some of the intellectually most interesting and potentially strongest candidates, such as Sen. Fritz Hollings, can hope for a better hearing than they otherwise would have had.

It is beyond the poor power of the Republican Party to create a "Republican era." Only the Democrats can do that, by nominating a candidate who takes them on an ideological bender. Kennedy might have done that.

But those who think that Kennedy's presidential prospects are dead as mutton should consider this: even in the year 2000, he will be just 68, a year younger than the current president was when, after several years' disappointments, his hour came 'round at last.