At Gorney Bravin + Lee, as I pored over Siena's drawings -- all done in colored pencil or pens on paper and illustration board, each about the size of a notebook page, most dated 2003 -- I breezily thought, "I could do these." After all, how hard could it be to fill a sheet of paper border-to-border with a repeating set of lines? I do it all the time when I'm on the phone or listening to lectures. It's mindless. Besides, Siena doesn't cover his tracks or fussy things up. If you examine his no-frills work, you can figure out the formulas he used to make them, then make one yourself.

Which brings me to the second thing. Two weeks after seeing Siena's show, aided by notes and sketches made there, and consulting the gallery's handy website, where all the drawings are pictured, I got some paper and colored pencils, and set out to reproduce a number of Siena's drawings. Gradually, as I either couldn't finish, lost my concentration, got mixed up, was unable to make things fit, or simply produced ugly renditions of what I was looking at, I grasped how much commitment and focus is necessary to make these little drawings and how incisive Siena's mind is.

Antenna, a set of open-ended concentric rectangles with a bar running from the center to the top of the page, looked especially easy. My versions were wobbly, slack, out of whack and bore no resemblance to Siena's. The shimmer of his marks; the way his line thickens and thins, goes wavy occasionally and bunches up on the left; how his hand grazes the paper, smudging areas just so; the way lines begin and end with no apparent jabs or fade-outs -- all these simple things were beyond me. A number of drawings involve interlocking-comb configurations. When I tried these, either my eyes blurred, my mind wandered or I fudged. Siena doesn't use rulers, so neither did I. But my lines were stilted and choppy and lacked the suppleness of his. Also, I pressed too hard on the paper or not hard enough, my colors were too bright or bleached out, and I continually neglected the faint background grids. Whatever, my drawings had none of the uniformity, luster, or ultimate order of Siena's. Mine devolved into discombobulated doodles.

My remake of T-Ramparts, a set of interlocking T shapes, was messy, not meshed. As for Siena's drawings of concentric rectangles or irregular interlacing grids, every time I thought I had it, I'd see that I missed things, got the sequencing wrong, or was essentially making things up. My versions of his drawings composed entirely of the numbers 4 or 6 were at sixes and sevens, spiraled into taut knots or coiled in ways I couldn't control.

Trying to copy Siena's art told me what looking at it more closely would have: Siena is simultaneously intuitive and controlling. He devises what he calls "visual algorithms" or "structuring devices," then toys with them. Saying he's interested in "repetitive modalities," he divides space, reiterates line and repeats patterns. In this, his abstraction is both factual and fanciful. Occasionally, but not here, he includes the specifications used to make a work. A simple one reads, "Curved, tapered line segments interlock slightly in succession as they circumambulate the picture (and do not touch) in an extended spiral ending at the center." This concentrated description yields an unexpected image that resembles a magnetic field of squiggly lines.

Although Siena's drawings aren't as rich as his paintings, they more than stand on their own -- even if many are unfortunately framed in gray. Seventy-eight drawings is a lot to take in; nevertheless, looking at the best of these beauties is alluring and intimate, like listening in on someone's thinking. Siena's art parallels Celtic manuscript illumination, Peruvian textiles, Myron Stout's circumspect drawings, Islamic tile work, charts, maps, tantric diagrams, African art and even the oddball investigations of Tom Friedman. As finicky as Siena is, however, he's not obsessive like Kusama, Samaras or Bruce Conner. And while his drawings are kaleidoscopic and bear a superficial resemblance to some of Louise Bourgeois's, they are never surreal.

As with post-minimalists and conceptualists like Dorothea Rockburne, Mel Bochner, Barry Le Va and Sol LeWitt, Siena, 46, contrives rules and follows them. Like Brice Marden and Terry Winters, he complicates these rules and works against symmetry and the grid. Sometimes Siena creates illusionistic space, as when he deploys elaborate lariats of line. Other times, as with the nesting rectangles, things flatten out. One 1985 drawing here is a helical shape that spells out the word nothing. Needless to say, I couldn't reproduce this image either.

Siena's drawings look easy but are worlds within worlds. The intricacy and intelligence of his work allow you to discern the nebulous possibility that the only thing as complex as the universe may be the human brain.