"Bellini, Titian and Lotto" at the Metropolitan Museum Realism is a shifty concept. What is real for you may be nonexistent for me - God, for example, or subatomic particles. The landscape of reality varies with time and place, and different versions can vie for dominance in the same hour and space. The Renaissance was such a moment, as the metaphysical reality of Christianity began to give way to that of empirical science and perceptual experience. You can see that happen before your eyes in "Bellini, Titian and Lotto: North Italian Paintings From the Accademia Carrara, Bergamo," a small, philosophically intriguing exhibition at the Metropolitan Museum of Art. Accademia Carrara, Bergamo, Guglielmo Lochis Collection Bellini, Titian and Lotto Moroni's "Portrait of a Little Girl of the Redetti Family" in this show at the Metropolitan Museum. "Christ and a Devotee" (1518), by Moretto da Brescia, on loan from a museum in Bergamo, Italy. It is not offered as a theme show. It is just a selection of 15 paintings, mostly of middling quality, on loan from a museum in Bergamo, Italy, that is under restoration. And the title may be misleading: Only one of the paintings is a Bellini, and the attribution to a young Titian of a weird small landscape illustrating the story of Orpheus and Eurydice remains uncertain. Viewers may connect the dots however they like. For me what is striking is the registration of a fundamental change in what European culture on the threshold of modernity would consider really real. In "Christ and a Devotee" (1518) by Moretto da Brescia, Jesus appears wrapped in pink drapery and holding a tall, heavy cross before a kneeling man in a black clerical robe in a pastoral landscape. Evidently the man's prayers have summoned Jesus into actual being. This is paradoxical: Jesus and his petitioner are painted with similar degrees of verisimilitude, but they are of entirely different orders of reality, one supernatural, the other earthly. The lesson is that Jesus is not just a figment of religious imagination but a metaphysically real being who comes to those who earnestly pray. Maybe the rise of realism in late Medieval and early Renaissance art was driven by a need to shore up belief in the reality of Jesus and his associates by picturing them as flesh-and-blood people. Add emotion and you have a potent inspirational brew, as in the one painting here by Bellini, from 1455-60, a bust-length group portrait of the Virgin and St. John weeping with red-rimmed eyes as they hold up the dead Jesus between them. Since the gods rarely grant mere mortals a visit, realist painting tells people what such an experience could be like. But if exceptional illusion-making skill is required to "prove" the reality of metaphysical persons and events, then is not doubt already lurking? A Crucifixion scene by Vincenzo Foppa from 1450 or 1456 is remarkably ambiguous in this regard. Jesus and the two thieves are bound to crosses in the rocky foreground of a gloomy, mountainous landscape, and the scene is framed by the elaborate, neo-Classical architecture of an arch-topped portal in single-point perspective. This is a puzzle: Are we supposed to be seeing through a doorway to another reality? Or are we looking with nonmystical eyes at an extravagantly framed mural on a church wall? Could this be a tipping point between a waning age of faith and the advent of secular humanism? Though not obviously, 16th-century portraiture runs on a parallel track. Early portraits of the period defined people in terms of their place in a divinely ordained order, extending down into the human social hierarchy. So in a portrait of Giovanni Benedetto Caravaggi by Giovanni Cariani from 1517-20, the subject's identity as a philosopher and doctor is indicated by external signs of blessedness: the thick book whose pages he turns; the satiny pink mantel and soft hat of the scholar; and an inscription painted above his left shoulder extolling his achievements. Similarly, in Lorenzo Lotto's portrait of Lucina Brembati from about the same time, the figure of a chubby noblewoman resembles a mannequin richly clothed and accessorized with jewelry, a sable stole and a big fur hat. The epitome of what you might call social formalism comes in "Madonna and Child With Saints Paul and Agnes, and Paolo and Agnese Cassotti," executed with lifeless polish by Andrea Previtali about 1520. Here the commissioners - a stolid, middle-aged man and his young wife - stand behind the divine group as if beholding a waxworks tableau. Mr. and Mrs. Casotti may have been exemplars of piety, but the painting says as much about the worldly recognition they aspire to as it does about their belief. This is religion in decline. The most vigorous coming trend in portraiture would focus on personal essence. In Giovanni Battista Moroni's "Portrait of a 29-Year-Old Man" (1567), the three-quarter image of a movie star-handsome man with close-cropped hair and beard is rendered with nearly photographic realism. More important, you sense an interior life animating his pensive, sidelong gaze. That young man is wonderfully eclipsed by the sitter in another work by Moroni: "Portrait of a Little Girl of the Redetti Family" (1570). A child of perhaps 5, she wears a gleaming dress of black and gold brocade, and jewelry of pearls and gold, but she is not just a fancily dressed doll. She has an uncannily lifelike presence and, you can't help feeling, a vital inner self. This is the kind of miracle that realists from Rembrandt to Thomas Eakins would embrace: that of the tangibly embodied human soul. Other, less immediately visible realities would impinge in the 20th century, but that is another story. "Bellini, Titian and Lotto: North Italian Paintings From the Accademia Carrara, Bergamo" continues through Sept. 3 at the Metropolitan Museum of Art; (212) 535-7710, metmuseum.org.