Have you seen those sneaky little drawings that toy with your sense of perspective? There's one, for example, of a beady-eyed old woman that, under intense scrutiny, turns into a demure, swan-necked ingenue. Cool is like that, constantly shifting under your gaze, as hypnotic as a mirage and just as evanescent. One second you're sure that you've found the quintessence of cool in Val Kilmer (with or without cowl). And then your focus slides and you're staring at Tim Roth (with or without scowl), a wholly different portrait of cool. A Zippo lighter stashed in a Prada bag. A knowing first novel that plays with the gauzy fabric separating language from reality. Hercules' latest, campiest incarnation. Cool is an attitude, backed up by achievement (see C-SPAN's Brian Lamb). Cool is someone who endures being labeled ''hot'' (see TV star-turned-movie star Lauren Holly). Cool is versatile: As a part of speech, it can be a noun, a verb, a modifier, an exclamation, and an article...of faith. The fact is: Nobody knows what cool is. Yet the fact is: Everybody knows what cool is. Cool, huh?