BEING THERE -- AMC Academy, Bradlick, Inner Circle, K-B Cinema, Pike, Vienna, Wheaton.

The sole joke in "Being There" is that the hero is not all there. He is mentally, and also sexually, retarded.

But by being there in another sense, being at the right place at the right time, he's able to win incredible power, riches and love away from everyone else.

The dubious taste of this premise is mitigated somewhat by the dignity with which Peter Sellers plays the role, and by Jerzy Kosinski's pointing the ridicule, in his screen adaptation of his novel, at the gullibility and self-delusions of those who are supposed to be mentally superior. The point, made cheerfully for the most part but occasionally also crudely, is that these people are so busy feeding their own egos that they fail to perceive what anyone else is all about.

The joke has remarkable staying power. Over and over, we see literal and naive remarks taken to be amazingly deep and philosophical. There must be two dozen examples of a flat statement about gardening, such as that things seem bare in winter but spring will surely follow, being received as an insightful metaphor about the state of the world, and each time we laugh at the trick.

Chance, the hero, has spent his entire life in one house, where his sole occupations have been gardening and watching television. Forced out into the world by the death of his patron, he's accidentally caught up in the lives of an extremely wealthy and politically powerful old man, his younger wife and their friend, the President of the United States. Covering Chance with admiration for his "honesty," "naturalness" and such compliments referring to his simple lack of guile, they gradually yield to him their control.

The reticent elegance with which Sellers plays Chance is contrasted with the frenzied way in which all but Melvyn Douglas, who does a masterful portrait of the old tycoon, play their roles. Shirley MacLaine, as the wife, and Jack Warden, as the president, seem to be so oddly placed in their positions of power as to lessen the value of Chance's achievement. If the point is that any fool could take over the country, there's no triumph in its also being accomplished by subnormal intelligence.

Nor do any of these people find it peculiar that the newcomer they so much admire is always watching children's television programs. If the movie wants to sneer at the quality of television, which it does with long blurry segments reproducing cartoons and game shows, it should take care to elevate its own world above a state of mindlessness.