First the name, Cleanth, a torture for a slight lad with glasses, introduced into the family by his great-grandmother who had a fondness for Cleanthes, the Greek philosopher. But it is no burden for Cleanth Brooks, now an important name in American letters.

At 8 tonight this critic of literature, a professor emeritus of rhetoric at Yale, will deliver the Jefferson Lecture sponsored by the National Endowment for the Humanities. It will be held at the Pension Building, by invitation. This annual lecture is the highest award given by the Endowment, and carries with it a stipend of $10,000.

The work by which he is best known to thousands, maybe millions, of Americans in school in the '40s and '50s (though it was published in a new edition as late as 1976) is a textbook written with Robert Penn Warren, "Understanding Poetry." It appeared in 1938, and to it many owe a fondness for verse better than routine schlock. He has also written a considerable amount on American literature, especially William Faulkner.

The typical American teen-ager after 12 years of inadequate and frequently barbarous education has as much use for poetry as for three extra toes, and while this should not be the case, Brooks and Warren (who, after all, were teachers) could not help noticing the truth of it. They undertook in their book, therefore, the bizarre task of civilizing them as much as within a mere book lay.

"When I think of those whose taste for poetry was ruined once and for all in the fifth grade . . .," he says now, declining to continue in a mode of horror.

He is a man of slight build with moon-white hair and eyes that appear gray behind thick glasses. At 78 he has substantial energy, perhaps inherited from his father, a Methodist minister who imbued young Cleanth with a love of books, though the lad at first wanted to be a soldier, smitten with the glamor of Nathan Bedford Forrest, Confederate general of Memphis. Later he thought vaguely of becoming a lawyer but was spared.

His central concern in the lecture will be the value of literature in an age of computers and moon rockets, and in a nutshell he thinks the chief value of literature is as the nurse of Wisdom. He does not claim for literature a cash value (though even today a certain number of careers stop dead because of ignorance, lack of balance and judgment, failure of sympathy and any number of other blessings that literature best of all might have conferred) or a practical value. Merely that it leads to wisdom.

Few things distress him more than statistics suggesting indifference to language -- he cites one, that 40 percent of American 17-year-olds can read a document and have no idea what the hell it says. (This may be because an opaque person has written it, of course, but Brooks is in no mood for comfort.)

"There are also those who say poetry is no more than the words on the page, with no particular relevance to reality; a kind of elegant game, self-contained. If this is so, then I've wasted 50 years of my life on poetry."

Born in Murray, Ky., Brooks attended a small grammar school out from Memphis at one point, where Latin and Greek were drummed in. It made no sense, of course, and later at Vanderbilt in Nashville he had more Latin and Greek drummed into him, but it was only at Oxford (Rhodes scholar 1929-32) that the painful verbs began to add up to Sophocles, say, and he saw there was more to Greek than small fellows on a Tennessee bench engaged in drudgery. He began teaching at Louisiana State University and, with Warren, managed the influential Southern Review, home to Allen Tate, John Crowe Ransom and others, from 1935-42. Five years later, he went to Yale.

"Fifty years ago," he says, "the breakdown in the teaching of language and literature was plain to be seen," but it did no good to complain, as many English teachers did. People said, well, those people are always complaining if their subject is not the sole aim and love of the world at large.

"At my first teaching post I quickly discovered the English department was regarded as a service department, patching up grammar of students going into really important things such as electrical engineering or biology."

Even if a kid learns to read, he may not read anything but junk and that is because, Brooks says, English is often so wretchedly taught. The poetry dished forth may be drivel and the teacher may have little love for or comprehension of better stuff, or no gift for leading youngsters to Castalian springs.

He spoke for a minute, in an interview, of a different sort of teacher, the late Urban James Peters Rushton who took on a tribe of callow yokels at the University of Virginia, though fresh out of Cambridge himself and scarcely older than they. In one instance among many, he converted a country boy from utter contempt to considerable awe of such poets as Yeats, Crane, Hopkins, in a matter of two weeks.

"But you can be sure, with a teacher like that, that he never made fun of a young person's ignorance. Never knocked them down, never showed condescension."

It is, when you come down to it, a question of love, he supposes, in the sense of wanting wonderful things for the youngsters ranged before a teacher.

His own method of tackling a poem, which he will speak a little about in his lecture ("I have to remember it's not a classroom or a seminar of critics, and I don't want to bore people; but I might touch on it somewhat") is to assume first of all that the poet really intended the words he uses. If he says cat, he does not mean dog or pet or feline. "Above the level of doggerel," he goes on, a poet means every word, and the reader's happy task is to think what the words mean.

And he gives the example of an anonymous Renaissance poem:

O western wind, when will thou blow,

The small rain down can rain?

Christ, that my love were in my arms,

And I in my bed again.

The poet, he says, uses "western" not because it has the right number of syllables but because the west wind is moisture-laden, gentle, bringing in the spring and so forth.

In his talk he will quote Yeats' "A Prayer for My Daughter," pointing out a few things in it that would be overlooked if it were read gloomily and half asleep, as one might read a newspaper report from Khartoum, not expecting to learn much, really. On the contrary, for poetry you need to think. When Yeats speaks of some laurel "deeply rooted in some dear perpetual place," you are not allowed to think that is just something vaguely pretty to say. Why "perpetual," why "laurel?" Perpetual has different meanings, different associations, from "everlasting," and laurel, the crown of poets and sacred to Apollo, is different from oak or elm or roses.

"It's like those Japanese paper flowers, all coiled up, and you drop them in the water and they bloom out," he says. It's a game, all right, but it's also getting to the gist of the matter. Undoing the fiery parcels, as you might say (or as Crane said) to see what's really inside. Novelists do it, usually at interminable length and commonly in tedious language, but poets do it tight, concentrated, and you warm it up right and mix it right in your head and utterly new wonderful flavors come forth from lines that might seem at first ordinary.

Brooks is generous when he speaks of others. He calls Yeats' poem "magnificent," and says "very great poetry" is being written in America now, some of it actually read, he thinks.

The decades and the centuries will sift it soon enough. In the meantime, if you fool with it at all, you might as well give it a fair chance, because (Brooks goes on) a real poet won't let you down; the more you push and shove at his words, demanding they justify themselves, the more they will reward you. But the work of a trite poet will disintegrate the minute you start tearing it apart. Magnificence in literature is, among other things, tough, and was designed to be rassled with, like Jacob's angel.

He does not deal much, tonight, with the curious thing about great poetry, that not only does it stand up to heavy handling and all the reader's challenges, for if that were so it might as well be science or logic or any other discipline involving sound argument and wide experience.

But poetry does far more: It makes its arguments with ordinary words that for no known reason take fire, lighting up not only the plain meaning of the verse but the whole room of the mind. Thus a reader can understand precisely how a fine poet gets his magical effects, and understand as much about meter as Milton did, without knowing how to write like him. In science if you understand it you can do it yourself. In poetry you can understand it and thank God for what you can never do.

Of those who wish to open for others the gossamer steel doors of the verse of our language, Cleanth Brooks is thought by almost everybody to be a dandy locksmith or, more loftily, on the best of terms with the Genie at the door.