Saint Nicholas.

It is hard, it is hard indeed," a bored theater critic intones in Tom Stoppard's comic masterpiece The Real Inspector Hound, "and therefore I will not attempt, to refrain from invoking the names of Kafka, Sartre, Shakespeare, Saint Paul, Beckett, Birkett, Pinero, Pirandello, Dante, and Dorothy L. Sayers." Shortly after firing this salvo of allusions at the derivative potboiler he is watching, the reviewer finds himself swept helplessly across the footlights, to his doom. Criticizing art is a dangerous business.

Three decades after Stoppard sentenced his critic to death by drama, two delightful new plays from across the Atlantic have picked up on the theme of aesthetic peril. Yasmina Reza's Art is cerebral and tightly woven, an inspired construct of ideas and banter; Conor McPherson's leisurely and enigmatic Saint Nicholas develops organically around a few evocative images. But both works comment incisively on the experience of being a critic - not a professional phrase-monger, necessarily, but anyone who wields an artistic opinion in the face of a hostile or indifferent world.

The protagonist of the one-man play Saint Nicholas is, in fact, a theater critic, a world-weary alcoholic whose enthusiasm for show business is less substantial than the foam on his beer. After years of churning out withering commentaries, this decaying Dublin hack (Brian Cox) feels both torment and relief when a play - or, more accurately, a player - finally makes an impression. Smitten with an actress in a mediocre production, he forsakes career and family, follows the lady to London, and, as one might expect, falls in with a crowd of...vampires.

Wait a minute! From a character study anchored in Dublin's theater district, whence the inexplicable detour through Anne Rice territory? Up-and-coming Irish playwright McPherson has acknowledged that the idea for Saint Nicholas came to him in a dream; not surprisingly, the monologue merges incommensurates with oneiric nonchalance. Lovers of form will scorn the work for its dreamlike shapelessness. But lopsided stories can have more charm - and even, in a way, more solidity - than narratives with Tiffany contours. Perfect shape smacks of insincerity, while a straggling plot invites belief, since it can have no other justification but truth.

And if any actor can tempt us to swallow McPherson's concept, it is Cox, a craggy, mesmerizing actor whose voice is so thrilling he could transfix listeners by reciting the ingredients on an aspirin bottle. (He was recently sighted in the films The Boxer and Desperate Measures.) Savoring the lines, delivering phrases with idiosyncratic but perfectly chosen shifts in speed, Cox dared the audience to doubt he was the wayward Dublin critic. Apart from a few chairs and milkcrates, the production (directed by McPherson) boasted no set, but the actor moved as if a tangible past surrounded him: a bar packed with wary actors, or dawn-struck vampires melting from a garden littered with bodies.

Cox has a supremely confident stage presence: at one press preview, he delivered a minute or two of the monologue while perched calmly on the armrest of a scribbling reviewer. The artistic authority he exuded underscored one theme linking Saint Nicholas's vampires to its thespians: the relationship between opinion and power.

Professional critics are vampires, the play suggests: they prey on creativity, and grow stronger by sapping artists' strength. Never self-sufficient, they revel in their ability to instill fear. McPherson even hints at how easy it is for critical pedantry to eclipse real understanding: a surefire way to escape vampires, it turns out, is to fling a few handfuls of rice at them. Compulsive connoisseurs of detail, the vampires (critics) will be glued to the spot, oblivious to the broader landscape, until they have counted every last grain.

Aesthetic verdicts wreak yet more havoc in Art, the brilliant comedy by French playwright Reza. Launched on the road to international success when Sean Connery's wife saw it in Paris in 1995, Art was recently a hit in London under the direction of Matthew Warchus, who is also responsible for the stylish, perfectly paced New York production. (The English translation is by playwright/screenwriter/director Christopher Hampton.)

It is hard to know which aspect of Art to admire more: the concept, or the execution. Reza has balanced ninety minutes of human and intellectual insight onto a witty premise: three men - devoted friends for years - find their relationship crumbling after one of them buys a white-on-white painting. The brooding Serge (Victor Garber) thinks the painting is brilliant. The irascible Marc (Alan Alda) thinks it is a joke. Yvan (Alfred Molina), always a moderate, comes down in the middle, to the fury of the other two. As the trio lurches ever closer to violence, the canvas becomes a window onto aesthetic, linguistic, and emotional abysses.

Capitalizing masterfully upon this conceit, Reza probes the ambiguities of art and affection in scenes that are taut, ingeniously structured, and often hilarious. Profundities skim by like ping pong balls. Does an art work have inherent value, or is value created by the market, or the era - or by a spectator's caprice? Is there such a thing as disinterested friendship? How about absolute truth? Are modern artists trying to pull one over on us? And is there any delicate way of disposing of olive pits?

Reza gauges the temperaments of her characters with a surgeon's precision, and the script's ever-changing currents of anger, resentment, and sympathy create astonishing suspense in a play that is basically one long discussion. The dialogue has the paradoxical versatility of an optical illusion: Serge, Marc, and Yvan criticize each other by criticizing art, and each betrays himself when attacking the other guy. And their quibbles over words and concepts (Is it the white in the painting, or the idea of whiteness, that is so upsetting?) could admit them to a deconstructionists' convention.

From the opening moments, when Alda peers at Serge's painting, tilts his head, takes off his glasses, steps back, and peers again, expertly-tuned performances by the three stars keep the intellectual debate churning beneath histrionics and deadpan comedy. Indeed, the production is a testament to the way set, lighting, costume, acting, and direction can work together to turn a script to best account.

Mark Thompson's stylized design gives the visual composition a tinge of antinaturalism. The set's stark, unnaturally high walls, dwarfing the actors, flavor the scenes with absurdism reminiscent of a New Yorker cartoon. Three contrasting chairs - one hyper-modern, one antique, and one somewhere in-between - serve as both furniture and symbol. (This set represents the apartment of each of the three friends in turn, with alterations of lighting and the substitution of a single painting making it echo the personality of each man.) Even the actors' matching costumes - blue shirts and ties beneath black suits, for example - subtly undermine the realism.

As Serge, Marc, and Yvan sputter over the tenets of art, the production calls attention to the fact that they themselves are fabrications. Think twice before announcing your critiques, it seems to warn the audience: you may yourselves embellish some tableau whose features you cannot perceive.