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. 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. [ABSTRACT FROM AUTHOR]