''A mind naturalized in antiquity,'' was the way the English painter Joshua Reynolds described Nicolas Poussin -- a carefully worded phrase which fits the seventeenth-century French painter like a glove. Even when he painted the ''Sacraments'' (and he painted two series of seven separate canvases, each canvas devoted to one ecclesiastical rite), he broke from usual practise by picturing them as events taking place in Roman times. Here were ceremonies of imagined early Christians in authentic settings, and the artist gave scrupulous attention to classical dress, to historical accuracy, and he based the figures in his dramas on antique sculptures and reliefs.
If this gives the impression, however, that he slavishly copied Roman and Greek carvings, it would be quite wrong. He chose to live most of his life in Rome, and he immersed himself in a knowledge of, and feel for, the remains of the older civilizations available at that period. It is this ''feel'' which makes itself evident in his deliberately composed paintings, and it seems to have been perfectly natural to his sense of the solemnity, nobility, and expressive lucidity required by his high art.
This ''Baptism'' (from the second series of ''Seven Sacraments'') shows Poussin's extraordinary ability to balance various needs which might have produced impossible conflicts in a lesser artist. His figures are measured and placed meticulously, but this does not prevent them from being full of life. Far from the excitable, ecstatic manner of the ''baroque,'' they express wonder, amazement, reverence, hope or scepticism as the symbolic dove appears above Jesus' head.
Through the four distinct groupings of people runs an energy of design which links and unifies so that every gesture, every motive or reaction, gains support or incentive from the others. In this way Poussin allows for both rhetoric (making it possible to ''read'' his narrative with unmistakable clarity) and for contemplativeness (making it possible to consider doctrinal significance, appealing to thought rather than sensuous emotions in the viewer). Inevitably the first tends to be restrained by the second, but it also, in the slow rationality of his art, means that he produces a very individual combination of momentary happening and permanent serenity.
In the ''Baptism'' he depicts a drama. The event is built up and intensified by the language of gesture and the positioning of the figures (and in this he is influenced by such artists of the Italian High Renaissance as Michelangelo and Raphael, as well as by antique sculpture). But at the same time he distances his world to a monumental remoteness. In all this he was ploughing an entirely different furrow from most of his contemporaries. His steady compositions had nothing in common with a Bernini or a Rubens. One writer has described his stylistic development in the 1630s as ''a solitary achievement.''
The second series of the ''Sacraments'' was Poussin's eventual answer to the persistent requests of a Parisian friend and patron, Paul Freart de Chantelou. Chantelou had asked for copies of the ''Sacraments'' which Poussin had already painted for another private collector (an unprecedented form of patronage for works of this subject), but after much dissatisfaction with copiers and many excuses on Poussin's part, the artist offered to paint an entirely new series. The ''Baptism'' was the third picture in it to be finished and was painted in 1646. The theme of ''Baptism'' apparently meant a great deal to Poussin: apart from the two pictures in the ''Sacrament'' series, he painted it four more times. He also made significant allusions to the ritual of washing and purification in several other works.
The more this ''Baptism'' in Edinburgh is studied, the more satisfactory its order and organization seem to be. Nothing is confused. Everything is poised and balanced. Proportion, mathematical exactness, piety and naturalness were Poussin's aims, and they are present with irreproachable propriety. But the pervasive light, the pleasant simplicity of the classical landscape (adapted possibly from a firsthand sketch he made of the Aventine), the wonderful spontaneity and pure lightness of the white dove itself, all add touches of poetry to the dominant prose of his style. ''Few artists,'' Anthony Blunt has written, ''can ever have set out to paint with such a load of doctrine -- ethical, mathematical, and aesthetic -- as Poussin, and it is proof of his artistic power that the works which he produced in this period should not have been pieces of dry pedantry. Happily he possessed the gift of transmuting his learning into an imaginative creation.''