BIOGRAPHIES are tricky. A good one must capture the essence of a personality, make it ordinary and accessible, while seeming all the while to focus exclusively on what makes its subject special, different, immortal. These two books document the lives of literary geniuses who were also prominent public figures. Robert Alter traces the elusive interchange between a life lived and the legacy of creative work it left behind. Jean Orieux simply trots through the scenery, resplendent as it was, of gossip and melodrama in 18th-century France.

Henri Beyle (1783-1842) and Francois Arouet (1694-1778) were both professional dilettantes, an intellectual vocation that has since gone out of vogue. They worked by improvisation. They held radical convictions about social justice and political reform that they occasionally betrayed for their own self-interest. Bourgeois iconoclasts, their conversational wit gained them entry into the chic aristocratic salons in Paris, and they each wove a living as writers out of the anecdotal and often subversive materials of their own experience. We know them by their one-word pseudonyms, Stendhal and Voltaire.

In the preface to his new life of Stendhal, Robert Alter says that he wanted to write "a biography in which the narrative and interpretive strands would be constantly intertwined," an enterprise he calls "critical biography." Jean Orieux, on the other hand, subtitles the recent translation of his 1966 Voltaire, ou la royaute de l'espirit "a biography of the man and his century." As their titles suggest, these two studies represent fundamentally different approaches to the same genre. Because Alter's succeeds so admirably where Orieux's badly stumbles, their publication raises intriguing questions about just what it is readers want from the biography of someone who is famous more for his work than for his life.

Robert Alter's A Lion for Love is a splendid example of a rare and difficult critical genre: a biography which chronicles and interprets the development of a character of complex imaginative genius, and at the same time produces a lucid critical reading of his works which is accessible to general readers, not just to Stendhal's "happy few." Alter understands that living and writing were absolutely connected for Stendhal. "Fiction," he points out, "became a form of self-knowledge" and that statement justifies Alter's biographical procedure. When he gets to the culminating reading of Stendhal's masterpiece, The Charterhouse of Parma, Alter Convinces us that the novel represents the apotheosis of both Stendhal's literary and his worldly experience and is a transcendent work that "brings one closer to the self-contradictory movements of a convincing individual psychology than any novelist before him had done."

As Alter puts it, "the material of experience was slowly percolating through the alembic of the self before it would emerge as art," and it is this filtering process in the development of the artist that Alter traces. A Lion for Love is a gentle book, a rigorous scholarly biography that remains honestly affectionate toward its subject.

Jean Orieux, unfortunately, does not employ Alter's synthetic approach. He tells a series of unrelated anecdotes, using his subject only in marionette-like fashion as a centerstage focus. Voltaire appears here as a figure in costume, a Regency dandy who amuses the ladies from his box at the Comedie Francaise by spouting tart aphorisms on cue. Each chapter opens with a list of Voltairean witticisms, a procedure that in itself distances us and falsifies the portrait we get. By trying to make his subject larger than life, Orieux diminishes him, one hopes unwittingly, into a surly caricature. His book serves as a lampoon against Voltaire as savage as any the arch-lampooner himself ever penned.

"I must pretend to be Voltaire himself," Orieux tells us, and he admits that this will not be a literary study. Still, we expect some insight into the qualities of mind behind the most barbed and adroit of Enlightenment intellects. We get instead no critical analysis either of the life or of the work, and an overabundance of unquestioning eulogy. Orieux's biography is both disconnected and repetitious, and it reduces Voltaire to a propagandist, a schemer, a popularizer and a scandal-monger.

Voltaire deserves better. He has repeatedly been dismissed as the most outmoded and the least enduring of the French philosophes. His ideas have been ripped out of context and treated like museum relics. Orieux gives us only court intrigue and bedroom farce. Voltaire cut a stunning court and salon figure, it's true, but in the end he cannot be separated from his writings, a life work that moves from Newtonian physics to history to religious tolerance and dramatic theory. Orieux's grandly choreographed panorama is finally both sensational and insensitive.

Neither of these books offers much that is new in the way of information, and both, curiously, follow on the heels of two fine studies: Theodore Besterman's massive standard biography Voltaire was first published in 1969, after the appearance in France of Orieux's book, and is now in its third edition; Gita May publisher her Stendhal and the Age of Napoleon in 1977, after Alter had already begun preparing A Lion for Love. Orieux merely adds to the already long list of ponderous, gossipy biographies of Voltaire, while Alter contributes a major critical study of Stendhal, the writer who exhorted himself, prophetically, "to work always for the 20th century."

Alter does what Orieux fails to do: he connects his subject to the dynamic shifts in post-Napoleonic France as Orieux neglects to make the connections between Voltaire's extraordinary political astuteness and the coming French Revolution.Alter engages his portrait of Stendhal with the new sense among writers that the progress of history could be translated into fiction and illuminated by art. It is, after all, the relationship between achievement and the texture of daily life that makes us turn to biography in the first place.