A tree grows in SoHo.

Alex Katz likes to paint, and his ambition as an artist seems to consist largely in not letting anything get in the way of his actually doing so, steadily and surely. As Katz expresses it, being an artist is as much an act of will as it is the product of inspiration or abandon. "When I work I don't ask, 'Does the world really need another painting?'" Rather, his attitude is, "Well, let's see what happens . . ."

I arrived at Alex Katz's studio midmorning on the first Sunday of spring, the only snowy day in Manhattan all year. He greeted me warmly, excited to get to the business of making his painting. When I had approached Katz with the idea of chronicling the birth of one of his canvases, I had no idea that the entire process would unfold in the course of a single day. I had expected to track his painting's progress over the course of weeks, even months, but it became immediately apparent that he was going to paint it now. He'd had a particular color in mind, one he'd seen back when he was an art student - a deep, crepuscular blue he'd attempted to capture about fifty years ago in a painting of a woman looking out a window. That effort failed, but after so much time Katz felt he had discovered the right subject for another try: a tree along a Pennsylvania road which he had sketched from his car one day last winter. He would arrive at the appropriate color later, while mixing paints for the second of two twelve-by-nine-inch studies for the work. He figured out the painting's dimensions and the precise shape the tree would take by reworking the image from his drawing using charcoal on large sheets of butcher's paper. These now lay on the floor to the left of a roughly seven-by-five-foot canvas, which was propped against the wall on a couple of gallon paint cans and gessoed to a glossy sheen. The tree's outline had been traced on the canvas weeks ago, in strips of greenish-yellow paint about the width of masking tape. He had mixed green in with the lighter color he normally uses in prepping his canvas so that the outline wouldn't immediately disappear under the initial coats of the darker paint he would be using for this work. The two oil studies stood in the studio window just to the right of the canvas. Next to them, taped to the pane, was the original drawing, the diffuse morning light filtering through it.

Within minutes Katz was pouring his blue mixture from a jar onto a sheet of wax paper. He swirled a five-inch brush in the paint briefly, then dipped it in medium and began covering the canvas with broad strokes of watery blue. As he worked we talked. Painting got easier for him, he said, "when I got rid of the idea that you had to be a genius to make a painting. I was just going to paint. I didn't worry about it any more. There was this huge weight lifted off me," he recalled, rounding his arms above his head in a quick gesticulation as though throwing off the frustration he had felt before his revelation. "I could just do what I wanted."

As Katz applied the layer of turpentine and paint in broad, easy strokes, my presence, perhaps, reminded him of the time the poet James Schuyler had been to his studio in the early '60s and had written about him as he made a painting. "When I finished the painting he was done with his piece. It was incredible. In New York the '40s and '50s were about painting and jazz; the '60s and '70s were about poetry." Katz was proceeding with his painting so matter-of-factly that asking anything about it would have been begging the obvious. He remarked that for him a painting had to be "modern," but that this didn't mean it was an improvement on anything that had come before, just that each canvas should be unique, "of its time." Such pragmatism is something of a Katz trademark, his way of not getting caught up in any thinking about paintings that would hinder their getting done. "I have to figure out a way to be partly unconscious, not to know exactly what I'm doing, so it [the painting] can show me." He seems to accomplish this by staying vertiginously busy with the task of transforming images from the outside world into aspects of his own vision. His preoccupation with the nuts and bolts of his task takes priority over academic considerations about his work. "Thinking gets in your way," he says bluntly.

Katz maintains a terrific, naive excitement that might not play well among those unused to hearing an artist exclaim of his own work, "Hey, that's great! Wow!" But he is so prolific that it must be impossible for him not to be continuously caught up in his efforts. By now Katz has observed many of his paintings over the course of decades, watching how they fare in various contexts, seeing the effects over time of the media he's employed. "They change as they dry," he mentioned. "They dry for years." At seventy-two, Katz can anticipate the results of his medium and technique with such confidence that he must often be the first to recognize the surprises a given work holds.

For Katz, making a painting is an athletic feat. He has seldom spent more than a day on any one painting, and never more than a day and a half - though every once in a while he might do a bit of "touching up here or there on a particular work." He recently made an enormous 28-by-10 1/2-foot painting (My Mother's Dream, 1997) in four sections - each a version of the same view at the same time on different days - as part of a series he calls "Environmental Landscapes." He finished it in about eight hours. He kiddingly dismissed my wondering whether this had been physically exhausting: "Naah . . . I'm a jock. Intellectually it can be very tiring, though." He arrived at this technique of building up to an all-out effort after trying several others, such as working on a number of paintings concomitantly over the course of days or weeks. "That never did it for me," he grimaced, reiterating that in the course of his career he has destroyed "thousands of paintings" that "just didn't work."

For those that do work his enthusiasm is boyish and infectious. Years ago, seeing reproductions of some of his works from the late '70s, I read into his style a shiny, chromatic polish, a sort of assemblyline vacuity reminiscent of Warhol. Katz's paintings weren't as wise-ass or nihilistic, but it wasn't clear to me then what risks, if any, they were taking. They seemed to occupy an artistic terrain somewhere to the rear of Pop's posturing - a rarified and civil atmosphere of neat lawns, elegant parties, and handsome people with impeccable contemporary tastes. A glance at his earlier paintings, or at his collages, would have prompted a closer look. In these, the lyrical intimacy and unabashed sweetness latent in some of the later series predominate.

Katz showed me one early painting he'd forgotten even existed until a friend recently asked him to repair it. It was an almost naively sincere folk study of a young athlete, a sandy-blond basketball player, ball in hands, wearing a look of eager confidence and friendly (if competitive) determination on his face (Basketball, 1957). He stood before a baby-blue background and was depicted with a jocular sensitivity to his all-American good looks which seems to have borrowed more from comic strips of the day than from traditional genres of portraiture. An appreciation of cartoons has, of course, been vital to many artists of the New York School: John Ashbery's poetic alter-ego Donald Duck, the collaborations of Frank O'Hara and Larry Rivers, and Joe Brainard's irresistibly perverse treatments of Nancy and Sluggo are all exemplary of that affinity. And the proud, easy humor of this early work prefigures Katz's own cartoons, which he has been drawing since the early '60s. It seems only natural that raw sentiment, which had become somewhat taboo in the heady, politically minded art milieu of the late '40s, should have been stumbled upon by a young artist with temerity and street smarts enough to continue exploring it as a source for powerful art. This stubbornness seems to have proven itself to be genius in disguise - likely the very genius Katz had to quit worrying about before he could achieve it. Katz's work remained marked by optimism and sentiment, even after the late-'60s ascendance of Pop, whose practitioners' cynicism, if deeply felt, was muted by a knowing humor and deadpan facture that contrasted with Katz's more painterly approach. "I realized early on that if I wanted to compete with Warhol and Lichtenstein, particularly Lichtenstein, I'd have to give up painting." Instead of competing on their terms, Katz continued to paint his own way. "I just had to keep going." His method could hardly be more dynamic and visceral, and it is more in the tradition of Pollock and Action Painting than the cool surfaces of many of his works suggest.

A number of paintings Katz had made over the winter were leaning against the walls of his spacious studio, which occupies half the top floor of a remodeled warehouse on West Broadway. Among the canvases was a large painting of an office building at night (Facade, 1998). While its subject is immediately apparent, the work consists simply of a black background on which have been applied broad, evenly spaced rectangular strokes of white paint to indicate the windows through which we glimpse the shapes of items in offices. Katz pointed out the brush he used to depict each window with the single stroke necessary to ensure uniformity; he'd had to nail two four-inch-wide brushes onto a strip of wood to arrive at a suitably large one. While he seems to have used a smaller brush and more white paint to depict some of the shapes within the offices, the interiors are for the most part suggested merely by how the paint came off the double brush as he swept it across the canvas. The unique "fingerprint" of each brushstroke proves sufficient to conjure in the viewer's imagination forms specific to each of the building's rooms.

Another recent urban nocturne, Bond St., 1998, features bright splotches to suggest streetlights and the headlights of cars reflected on the avenue. While this painting is also largely black, Katz's use of murky halftones to suggest curbs, reflected light, and buildings infuses the picture with the various depths of real night. The pictorial clarity of these recent works masks the swift, painterly, almost abstract gestures that have been so effortlessly employed to achieve it.

Soon Katz started filling in the spaces around the tree with the blue paint. We spoke less. He grew more focused, as now each stroke would contribute to the look of the work when it was finished. His movements were methodical. He'd cover one section evenly and move promptly to the next, clockwise around the outline of the tree, until he had all the gaps filled in. There was no talking at all between us by the time he changed brushes to begin work on the tree itself. I had no wish to play the man from Porlock to Katz's Coleridge, and, anyway, just watching him paint had suddenly become a dramatic pleasure. His engagement with the canvas had grown intense. He used a small stair-step to reach its top, pushing the steps deftly to the side as he worked on lower portions of the painting. The tree began to look muscled and sculpted, its lineaments recalling a woman raising her arms exultantly to the sky. Katz defined the tree's edges with long, careful strokes of a one-inch brush; he then finished this edging by smoothing the paint with some quick back-and-forth brushwork, the sort of detailing a housepainter is familiar with. At the bottom of the painting he began to give form to a wooden fence behind the tree. When he had completed this stage of his work, Katz walked away from the canvas for the first time. He'd been painting for about an hour.

The tree was in place, painted a blue just shy of purple. It stood naked in front of a fence, against a background of barely brighter, winter sundown blue. Katz walked around, pausing at various points in the studio, twenty or thirty feet from the work. He had yet to depict the tree's limbs and to refine the wooden fence.

When he returned to the painting he began effecting limbs, using the same dark blue. He made rapid, single strokes emanating outward from the tree's upper branches. His motions were determined and sure. When he got to the bottom of the canvas he painted similar, vertical strokes, which could read as strands of high grass, saplings, or weed stocks around the tree's base. After about forty such swift, controlled movements - between which Katz would occasionally step to one side of the painting or the other, ducking, arching, and stretching to look at it from different angles - he dashed off one mark in the lower left of the canvas that seemed out of place. It looked too high and far forward to be a stalk, and it obviously wasn't one of the tree's branches. He slowed, marking only a bit more grass, then stepped away from the painting again, this time for a longer while.

He walked to the far wall and adjusted the blinds in one of the many windows in his studio, registering the effects of this slight change of light on his work. He prowled back and forth, keeping his eyes trained on the painting, nearing it, striding backward from it, turning - his head craned over his shoulder as he walked away - then turning again to consider it from another distance, then another. He'd put his hands on his waist, or, folding his arms lightly, touch his thumb and forefinger to his chin, intently studying his work, his brow furrowed by how wide his eyes were open. "I'm close," he said suddenly, grinning. "Only a little more." After some additional marks, a few halftones here, one there - "this is something new I've been trying in these recent paintings" - Katz declared he was finished. Then he picked up his brush again and made another few strokes, then another touch-up. "It's there." Katz finished the painting some six or seven times in the minutes before he finally rinsed his brushes out, always returning to it for just one more quick daub, to add a small limb or some detail on the ground. "That's it . . . It's hard to know when to stop, when elaboration causes it to lose energy. It's a trade-off?' Each time he returned to the work he risked all the preparation that had gone into it, as well as the effect which, after fifty years, he knew he was finally realizing. With every gesture he watched his work change "completely," as he put it. One bold stroke of his brush and he would exclaim, "That really opens it up!"

We talked as he wiped up the few splotches of paint he'd spattered on the wall. He told me that his focus was twenty feet behind the canvas as he worked, precisely the distance of the arc I'd traced behind him as I walked from side to side, trying to get the best view of the painting as it came together. He showed me more of his works. The large ones we looked at from the farthest distances his loft allowed. He pulled out a tall, fatigue-and-aqua-drab painting of the surface of water in a Maine sound on which a dock is reflected (Pier, 1997). An image similar to this had stayed with him since his days in the navy, but, like the blue from his student days, he'd been unable to capture it in a painting until recently. Another painting (Brown Woods, 1997), a large, rust-brown landscape, opened up across the length of his living space. It was of trees in a forest whose floor was eerily sun-dappled. Pine limbs seemed like tracings left by electrical particles whizzing about the dank under-canopy.

I asked Katz what kept him going. "It's an issue of character," he replied. He mentioned another artist, the first of his generation to "score." "He only had four shows. His first was great. Everyone was wild about it. The second was good, the third one was okay, but by the fourth one he'd completely lost it. No one ever heard from him again . . . You can't get too discouraged. You just have to keep at it and do things," he paused, "in your own time." Keeping us company in his living room was his cutout of Rudy Burckhardt and the late Edwin Denby, poised in wooden folding chairs, facing one another. Ada, Katz's wife and the subject of many of his portraits, perhaps sensing that we were "coming down" from the morning's experience, made sure we were comfortable, asking occasionally if we needed anything. As we roamed the studio I referred to Eric de Chassey's catalogue essay for a recent exhibition of Katz's work in Paris. "Yeah, I liked that," Katz said, giving his Yankee paraphrase of this rigorous article's conclusion that he was living up to his end of Rousseau's social contract: "That's what I've always sort of thought I was about. You know, doing what you want without breaking the law." For Katz, this clarity of conscience is liberty, and it is the bond that unites art and mere living.

I took one more look at the painting I'd watched him make. The errant mark had resolved into something subtle and mysteriously essential to the finished work, a hint at this artist's exuberance, an indelible trace of the risks freedom implies.

What had happened was another painting.