While viewing the magnificent Poussin exhibition at the Metropolitan Museum, I was reminded of Saint Thomas Aquinas’s assertion that faith requires more than certainty, it also requires an act of the will.

Unlike so many of the wildly talented Italian painters of his day, who almost could have created their pictures blindfolded, Nicolas Poussin, who was born in Normandy and lived, mostly in Rome, from 1594 to 1665, seemed to will his compositions warily into being, as if no pictorial element should be allowed to escape the tutelage of total consciousness. Doubtless aware of his manual awkwardness, and also of his superiority as a designer, Poussin became the archetypal cerebral painter.

As the current exhibition reminds us, however, he was also among the earliest French painters to think of his art as a vehicle for emotion, like music, and he even attempted to introduce into his art the ancient Greek musical modes as governing registers of feeling. The show, on view through May 11, assembles about 40 paintings and as many drawings, all of which embody a response—now passionate, now meditative, now melancholic—toward the Mediterranean landscape.

Its title, "Poussin and Nature: Arcadian Visions," might give pause if it were intended to ram home a dogmatic art-historical viewpoint. I don’t think that’s the case, however, because its organizers clearly had a different purpose in mind. Keith Christiansen, the Met’s curator of European paintings, to whom we should be grateful for the securing the loan of these masterpieces, and Pierre Rosenberg, the Poussin authority and Director Emeritus of the Louvre, are addressing not the experts but the public at large. If you think of Poussin as an arid French intellectual, they seem to be telling us, please look again—consider this tremulous, swaying foliage, this delicate play of light, these tender distances with their meadows and streams and sunstruck campanili. And of course they are right, and fully in line with an established view of Poussin as a super-sensitive landscape artist. Long ago, both of the preeminent postwar Poussin specialists, Anthony Blunt and Walter Friedlaender, stressed the artist’s gifts in this field. "It is true that his finished paintings of landscapes are as rational in their presentation as his figure paintings," Blunt wrote, "but, as with them, his generalizations are based on profound knowledge of nature as he saw it …"

The question is: What do we mean by "nature?" Between Poussin’s period—the period of Galileo, Kepler, Fermat and Descartes—and our own, an enormous shift has taken place, especially for English-speaking people. For the early 17th century, "nature" meant the physical world as expressible in mathematics—what we nowadays usually call "the universe." For us, though, "nature" has Wordsworthian overtones, since in the very word we hear always some echo of the idea that the human spirit vibrates in consonance with floral and geological structures, as if forests and mountains were sentient, imaginative beings in themselves.

There is plenty of 18th-century painting in which we already detect this notion—we may think of Pierre-Henri de Valenciennes, whose proto-Romantic plein-air oil sketches were rediscovered long after his death, and perhaps we feel a bit of it, too, in the landscapes of Poussin’s colleagues Claude Lorrain and Gaspard Dughet, Frenchmen at work in and around Rome. However, Poussin’s reputation as a lyrical devotee of natural beauty as such, rather than as a rationalist student of the laws of optics, cast shadows, aerial perspective and so forth, has been undercut, not reinforced, by the reattribution to other hands of a large group of landscape drawings previously credited to him. These mostly quite beautiful drawings, the so-called G-group, are actually displayed in the present show, which leaves it rather at cross-purposes with itself. (Poussin fans who do not follow such matters will be amazed to learn that "The Aventine," a contre-jour study of that Roman hill from across the Tiber and one of the greatest landscape drawings ever made, has been tentatively demoted.)

It is perhaps telling that in Queen Elizabeth’s huge collection of Poussin drawings, at Windsor Castle, whose provenance is mercifully unimpeachable, not one landscape drawing may be found. Poussin’s great paintings of figures in landscapes have wonderful pastoral passages, but they are not exactly paintings of nature. It is not so much that they often focus on figures or architecture, or that the trees are botanically unidentifiable—generic oaks at best—but rather that everything arboreal tends to be schematically handled as the painter tackles grand metaphorical themes. If you scrutinize Poussin’s townscapes, you discover pieces of keenly observed, naturalistic tonal painting that are only a step away from Corot, especially with regard to the quality of the light. The foliage is not analogous, however, being generally rendered according to conventions typical of stage or tapestry design, in which the nearer boughs are simply painted lighter and the farther or lower ones darker. And really that is all Poussin needs. Wary of distractions, he rhythmically masses his greenery to set off the main subject of his pictures.