Mondrian would have liked it

Quietly, and without fanfare, Peter Poskas has moved into the front rank of living American landscape painters. He has done so by producing exquisitely composed, precisely representational pictures of the American countryside and a few urban areas.

At first glance, his paintings of rolling hillsides, grazing cattle, farms under snow, and rickety porches appear to be nothing but straightforward depictions of actual places, with nothing altered and nothing rearranged.

That, however, is not the case. Everything in his compositions may be carefully drawn from life, but the arrangement on the canvas results from the most careful redistribution of elements, the most sensitive adjustments of color to form, texture to line, volume to space, and tonal relationships to a picture's overall atmosphere and character.

Objects are moved closer together or separated entirely; doorways, drainpipes , and railings are heightened, dramatized, or eliminated; clouds are moved about; solid surfaces are textured; and light itself is subtly altered - all to create the most perfect and most satisfying illusion of reality, of truth.

It is a perception of truth, however, that respects order as much as verisimilitude, and that pays tribute to such strict masters of pure form as Mondrian and Gris. As a result, every dot, line, mass, detail, and color must serve two demanding masters: faithfulness to appearance and ideal order - and must do all it can to reconcile these normally contradictory goals.

In this, Poskas joins the ever growing - if underpublicized - band of artists whose creative stance is conciliatory rather than inflammatory, who prefer to bring together the extremes of emotion and form in art rather than polarizing them. These artists work in various styles and have done some remarkable things, but they and their work have never quite received the attention they deserve. The art world, unfortunately, tends to focus on those who shock or surprise, on those who push art into new directions or who have manufactured new sensations.

Recently, however, the moderates have increased in number, and a few have begun to receive serious recognition. Peter Poskas is among this group. His growing success - he recently had a museum retrospective, will have a New York one-man show in October, and is scheduled for another museum exhibition in November - would have been impossible 15 years ago and unthinkable 15 years before that. At that time, his work would have been ridiculed as irrelevant, as not really art at all.

Those days, it is to be hoped, are gone for good, and with them the notion held by a few in the art world that painting and sculpture must rigidly conform to a particular ideal or dogma to be considered art.

The paintings of Peter Poskas deserve respect, because they are among the most intelligently conceived and sensitively executed rural images being produced today. There aren't many artists who can synthesize the accidental and the intentional, the ''natural'' and the formal, so brilliantly and convincingly - and end up with pictures that are true to life and to the ideals and values of art.

Even that, however, cannot guarantee artistic significance. Something else is needed that will permeate every atom of the composition and stamp it as absolutely authentic. In Poskas's case, it is his handling of light. Everything his brush touches radiates with a soft, subtly luminous glow that transforms even the most commonplace subjects into art. It is present in every one of his paintings, even in his smallest oil sketches and coldest winter scenes.

In some ways, that light is what ''Afternoon Before the Frost'' is all about. Everything is cool, crisp, and expectant. The wintry light is evenly distributed throughout, and there is definitely a chill in the air. Even the massed foliage of the plant at left and the few flowers we can glimpse through the window cannot dispel our feeling that the first frost of the season is on its way.

This effect is heightened by the severely geometric composition. Mondrian would have appreciated Poskas's orchestration of verticals and horizontals and his shrewd placement of window and post. The latter two appear ''correct,'' and yet in every depiction of this porch - it is one of the artist's favorite subjects - they've been altered or moved slightly but significantly to conform to other formal ideas Poskas wanted to try out.

His ability to find variety in the same subject is truly remarkable. I believe he could devote his life to painting only this porch and what is on or around it, and do so without repeating himself or producing a dull picture. Not many artists - past or present - could do that. And yet, to a large extent that is what art is all about.