Aside from his massive intellectual journal, the ''Zibaldone,'' Leopardi's major prose work is ''The Moral Essays,'' which he composed for its first edition entirely between Jan. 19 and Nov. 16, 1824, when he was 26. As early as 1819 he had written his first note to himself on the project, and in 1820 he mentioned it in a letter: ''Recently, almost as if to revenge myself on the world, and also on virtue, I have thought up and sketched out a few little satirical prose pieces.''

Leopardi felt that Italian literature was deficient in satire, and when writing ''The Moral Essays'' he decided to imitate Lucian, the Greek satirical writer of the second century A.D. who wrote comic dialogues between sham philosophers, mock eulogies of astrologers and thinly veiled personal attacks on The Mistaken Critic, The Professor of Rhetoric and The Ignorant Book Collector. But Leopardi soon abandoned this model. He was never abusive to individuals, his dialogues were seldom genuine debates and his tone was never savage. Indeed, he was far more abstract and sober than Lucian in the presentation of his ideas, which unfold in brief, interrelated discussions of moral philosophy (''moral'' in the sense of ''how to live'').

AFEW of the titles of his dialogues give a sense of his austere whimsy: ''The Dialogue of Fashion and Death,'' ''The Dialogue of an Imp and a Gnome,'' ''The Dialogue of Fredrick Ruysch and His Mummies.'' In order to indicate the eternal and universal validity of his arguments, Leopardi based his dialogues on ancients and moderns, allegorical figures and real people, Europeans and exotics (an Icelander, for example), the educated and the average person (''The Dialogue of an Almanac-Pedlar and a Passer-by''), even on animals (''The Canticle of the Wild Cock'').

In constructing his fables, Leopardi was at once uncomplicated and ingenious. Fashion and Death, for instance, are sisters, both daughters of Decay. Fashion likes ''to pierce ears, lips and noses, and to rip them with the knicknacks I hang in the holes.'' And out of respect to her sister, she has ''never allowed the practice of dying to fall anywhere into disuse.'' In the dialogue between Columbus and a member of his crew, the discoverer justifies the voyage to a frightened crew member by saying that ''for a while it keeps us free of boredom, renders life dear to us, and makes us value many things.'' This stimulus is necessary to create even the most transitory pleasure since, as Leopardi states in another dialogue: ''As you necessarily love yourself with the greatest love you are capable of, you necessarily desire your own happiness as intensely as you can; and never being able by a long way to be satisfied in this craving, which is supreme, it follows that you can in no way avoid being unhappy.''

The cosmic pessimism Leopardi embodies is both subjective and objective. Subjectively, human happiness demands ceaseless pleasure of the most intense sort, which is not only practically unlikely but psychologically impossible to sustain; objectively, the human species is crushed again and again by the forces of nature (including human nature). In ''The Dialogue of Nature and an Icelander,'' nature announces matter-of-factly that ''The life of this universe is a perpetual cycle of production and destruction.'' Before he is consumed by two hungry lions, the starving Icelander asks, ''Who is gladdened or who is benefited by this most unhappy life of the universe?''

ONLY illusions, such as the pursuit of fame or love, can lend a spurious and fleeting joy to an existence otherwise consumed by noia, the yawning boredom Leopardi so greatly feared, an anguish comparable to Sartre's nausea. Characteristically, in ''Parini, or Concerning Fame,'' after systematically ridiculing the pursuit of glory and proving its laurels are weeds, Leopardi turns around and urges the gifted young writer to seek fame after all. Since ''nobility, warmth and fecundity of heart and imagination, are of all the qualities that fate bestows on human spirits, the most harmful and the most to be lamented,'' the hapless genius should at least enjoy the mixed blessings of fame. Mr. Creagh remains true to the eccentric punctuation in the original Italian that functions so beautifully as a somber retard in this essay's final sentence - a sentence, moreover, that could well serve as Leopardi's motto: ''Great writers, incapable, by nature or by habit, of many human pleasures; bereft of many others by choice; not seldom neglected in human society, except perhaps by those few who pursue the same studies; are fated to lead a life similar to death, and to live, even supposing they attain to this, when dead and buried. But our destiny, wheresoever it may lead, is to be followed with a spirit strong and great; a thing above all demanded of your talents, and of those who resemble you.''

The ambiguous rewards of glory came to Leopardi only after his own death. How just that this tragic figure should have turned out to be the representative modern poet of the joyous Italians, on the principle that each nation gets the poet it least wants or deserves - the principle by which the ruminative, soul-searching Russians get the concise, elegant Pushkin; the prudish English get Byron; the neurotic Germans get the supremely healthy Goethe. But of course artists, as Leopardi remarked, take a fierce relish in revenging themselves on the world.