Who is brightest -- and sometimes best?

A brave professor at Yale, Robert J. Sternberg, has asked himself, and a lot of experts and laymen, a dangerous question: What is intelligence?

Definitions quoted by Professor Sternberg in his report in Psychology Today magazine keep contradicting one another. Intelligence is ''the ability to carry on abstract thinking.'' On the other hand, intelligence is the capacity ''to adjust'' to one's ''environment.''

The 140 experts, being academics, tend to regard ''reading skills'' and ''verbal fluency'' as the key indicators. The 186 laymen prefer evidence of ''practical problem-solving ability.''

So on your trip cross-country, would you rather have in the passenger seat a chap who can crack puns and reel off Walt Whitman, or a silent genius who knows how to fix the carburetor if you break down in the Mojave Desert?

The question gets complex, as people say who hope to be thought intelligent.

On the subject of intelligence, as on most subjects, opinions are less crisp than five decades ago when Edward L. Thorndike proclaimed his confident, no-nonsense definition of intelligence as ''that quality . . . in respect to which Artistotle, Plato, Thucydides, and the like, differed most from Athenian idiots of their day.''

Almost all statistical measurements of intelligence, accepted at their face value 25 or 30 years ago, are now topics of fierce debate. A professor at Harvard, Stephen Jay Gould, has written a scathing critique of IQ testing in his new book ''The Mismeasure of Man.''

We are also beginning to appreciate the prejudices that lurk behind most definitions of intelligence. The dude with ''street smarts,'' standing on the corner of Broadway and 42nd, has a problem respecting the country intelligence that knows the side of the tree moss grows on. The young are bound to be skeptical of the intelligence their elders fondly call the wisdom of experience. It is very hard for the rich not to think of their wealth as a proof of intelligence -- no matter what that makes the poor.

The cruelty inherent in every definition of intelligence is that it leaves somebody else looking less intelligent, when, in fact, he or she may simply possess a different kind of intelligence. We are beginning to understand this -- to become a little less elitist on the subject of intelligence -- and that is good.

Still, profound confusions continue to persist about the nature of intelligence. Pick up any popular scientific magazine and you are likely to find intelligence proferred as a four-color cutaway chart of the brain. On this marvelous neurological switchboard, it is often assumed, behavioral engineers are free to place their calls in the interests of Pavlovian conditioning.

Or perhaps, if the author is a Freudian, intelligence will be presented merely as the rationalization the superego works out as a rather nasty compromise with the id.

Or if the metaphor of the computer entrances the definer of intelligence, ''what we call minds are simply very complex digital computer programs,'' as one summarizer has put it.

In these prevailing theories, intelligence is at the mercy of its own pushbuttons -- a mechanism with little or no sovereignty.

At this point, the Sternberg study produces a surprise. The laymen -- though not the experts -- stubbornly insist on listing ethical standards among their criteria of intelligence. For example, ''insufficient consideration for others'' is declared to be a conspicuous sign of lack of intelligence.

Everything that is intelligent, it seems, finally longs for intelligence to be something more than a salivating reflex, or a print-out, or a pseudo-civilized suppression of greed, hate, and lust.

Is it whimsical or sentimental to give intelligence a moral dimension? Hardly. The root meaning of intelligence goes back to the Latin ''inter'' (between) and ''legere'' (to choose). Intelligence, in its literal meaning, describes the capacity to ''choose between'' good and evil.

The single fairest test of intelligence at the moment may follow from that definition:

Can human beings make the choices to avoid their self-destruction through World War III or massive abuses of planet earth?

A passing grade will do.