Some people think of television as a planet on which intelligent life has yet to be discovered. But Mortimer J. Adler, who for 50 years has been trying to make mind exercise as popular as body exercise is now, sees television as an appropriate medium for reeling off Things Worth Knowing. And so, tonight at 10 on Channel 26, Adler and Renaissance-man-about-town Bill Moyers begin their new six-part public TV series "Six Great Ideas" that runs through November. It grew out of a 1981 seminar sponsored by the Aspen Institute for Humanistic Studies in Aspen, Colo.

Adler -- an impressively energetic 80 -- plays host to 14 invited celebrity thinkers, but on tonight's "Truth," Adler does most of the talking himself, whether being asked by Moyers "one of the oldest questions of all, 'What is truth?' " as the two of them sit near one of the great brooks of the Western world, or holding forth in the seminar room on such topics as objective vs. subjective truth and the definition of a rational mind.

The series could perhaps be thought of as jogging for the brain. In addition to truth, the ideas to be considered are beauty, goodness, liberty, equality and justice. What makes them great, beyond the fact that Adler says so? Actually, Adler pretty much thinks Adler saying so is sufficient proof. During a recent visit here, he said, "I assure you in the case of these six ideas, the simple evidence for that is, you can't go through one week of your life without using, at one time or another, all of those words."

Produced and directed by Wayne Ewing (on film, for some obscure reason, rather than the logical thing, tape), The Mort and Billy Show is just filmed talk, edited down to six hours from at least six times that much raw footage. "One of the reasons I like working with Bill Moyers is, there's no gimmicks at all," Adler says. "I'm an author, I'm a writer, but I think what happens in these films will, for many people in this day and age, awaken interest in the ideas the way the written page wouldn't have. It's livelier. In some sense, the viewer can identify with Bill. He's a common-sense fellow--serious, moral, practical, asking me questions that the viewer wants to ask."

Adler has little patience with anybody, but in particular with skeptics who think few TV viewers will sit still for talky thought-provokers about truth and beauty, where the only names dropped are names like Pericles, Copernicus, Mozart and Jonathan Swift. "I think we're going to get quite an audience," Adler says confidently. "I don't think we're going to get the audience of 'Monday Night Football.' We don't want the audience of 'Monday Night Football.' I would like between 5 and 10 million viewers, which I think is very good indeed. More than any book on the subject would reach."

Intellectuals, noses in air, notoriously look down on television--a difficult feat when your nose is in the air, actually--but Adler springs to its defense. "Look, I think nine-tenths of the 40,000 books published every year are crap," he says. "They shouldn't be published. Television hasn't got a worse record than that. Quite seriously, the publishing industry makes a social, educational contribution by the relatively few good books a year published. Well, there are a few good television programs a year, too. I don't see any difference. Sure, most of the stuff is tawdry, vulgar, trivial. So? Think of the paperbacks in the grocery store, the drugstore. Lousy! They're just as much mind-killing as the bad TV is mind-killing. What's the difference?

"My objection to kids watching television," Adler says, "is not so much that it's a waste of time, but that they form the habit of passive reception. That's what's wrong. There's nothing wrong with the medium; it's the misuse of the medium. And if you ask me what single thing I praise about these six films that Moyers and I made, it's that they won't let you do that. You can't watch them passively. You watch them passively and you won't see them at all."

Television, Adler thinks, may be the most noticeable mass mediocrity machine in our culture, but it isn't a lonely little polluter in a forest of lofty giants. "I've been in this business a long, long time and I know I'm right about this," says Adler, who can't be said to lack the courage of his convictions. "The publishers, the editors of newspapers and television producers, all of them, grossly underestimate the intelligence of the American people.

"I've been on the lecture platform for 60 years, and I have always, in every lecture I have given, aimed it over the head of the audience. Not so far over their heads they can't reach up and catch something--just so they have something to reach for. And they deeply appreciate it. When you talk down to an audience, they know it right away and resent it."

For six weeks on PBS, Mortimer Adler, Bill Moyers, and 14 other smart cookies will be talking up, not down, and despite the high hot-air content of these shows, it's one of the rarer forms of hot air to make it onto television.