Musicians evolve at the risk of losing their audience -- fans always prefer known quantities to variables. David Bowie is one performer who's made a career out of defying rock convention so flagrantly and with such finesse that fans not only forgive him for it, they worship him. From the fey folk of his pre-Space Oddity days to the right-angled, martinet funk of "Fashion" and Tin Machine's bloated hard rock, Bowie's only constant has been his inconstancy. Considering his vast, diffuse output -- three dozen or so albums and counting -- you can hardly fault him for the occasional bit of slag, especially when there are more than enough nuggets to make all the panning worthwhile. At every career turn, Bowie has often sustained his flashes of brilliance -- The Rise and Fall of Ziggy Stardust and Hunky Dory are two examples among many. His Janus ability to perform with utter candor (as on "Rock 'n' Roll Suicide" and "Quicksand") and then, in the blink of an eye, don a mask and enter a role (witness the plastic soul of Young Americans) makes Bowie a living puzzle. We're still awaiting all the pieces.