Several years ago, I had a discussion with a songwriter friend who’d become convinced that it should be possible to create a set of mathematical formulae that would allow a computer to determine the genre of any song. You’d just input the crucial data — beats per minute, instrumentation, tone of singer’s voice, verse-and-chorus structural elements — and the computer would spit back a label: R&B, Britpop, heavy metal, gangsta rap . . .

My friend had been pushed to this rather desperate point by a major-label A&R person who’d complained that several songs my friend had written were "too metal." This was before the OzzFest-driven metal resurgence, so "too metal" was bad. But my friend felt he’d simply been writing "heavier" pop material. So he asked me a crucial question whose answer he already knew: did I consider AC/DC and Aerosmith, two bands he knew I liked, heavy metal or hard rock? My answer: "hard rock." He responded that my choosing "hard rock" over "heavy metal" was an elitist rationalization. Since I didn’t care for metal, I’d simply found a more palatable category.

The answer to the question of how artists, critics, and fans distinguish, for example, one subcategory of rap from another, might be difficult to articulate, but most of us instinctively just know. You hear a song, look at the band, and in less time than it takes to say "subgenre," you’ve ingested countless audio and visual cues to get your answer. There will always be arguments among friends as to whether a band are this or that — arguments that ultimately boil down to whether a band are "cool" or not. But mostly we agree on what’s what, and when we can’t, we invent a new subgenre that smoothes over the discrepancies. And when the music industry gets lucky, that opens the door on a lucrative new trend like "grunge," which was really just non-metal, punk-inspired bands playing variations on pop metal (Nirvana), classic rock (Pearl Jam), or pre-Van-Halen-style heavy metal (Soundgarden).

I’d forgotten about that discussion with my friend until recently, when the 30th-anniversary two-CD edition of David Bowie’s classic The Rise and Fall of Ziggy Stardust and the Spiders from Mars was issued by EMI in all its digitally remastered glory. Packaged neatly as a CD-sized hardcover book or novel, the reissue features a lengthy and informative essay by David Buckly that’s bookended by the two CDs. The first disc is the Ziggy Stardust album from start to finish; the second consists of 12 other tunes that have come to be associated with Bowie’s Ziggy period, including "Velvet Goldmine," the single versions of "Moonage Daydream" and "Hang On to Yourself" that were released under the alias Arnold Corns prior to the Ziggy album, an amusing (if not quite good) cover of "Round and Round," and acoustic demo versions of both "Lady Stardust" and "Ziggy Stardust." (What’s missing, in my opinion, is at least a live recording of Bowie’s Ziggy cover of the Velvet Underground’s "White Light/White Heat," which was a staple of the Spiders from Mars’ live set.) There’s also a new mix of "Moonage Daydream" that does little to improve upon the original.

What interests me most about the new Ziggy is the stark difference between the demo version of the title track, which sounds like a folk-pop number, and the Mick Ronson–driven "Ziggy Stardust," which, if I’m not mistaken, has some heavy-metal overtones that we’re all familiar with from the album. Not that there’s anything wrong with heavy-metal overtones. However, in his accompanying essay, which does well to dispel the lingering misperception that Ziggy is a concept album or rock opera, David Buckly points out the wide-ranging impact Ziggy Stardust has had on other artists who have come along to create new genres (or, at the very least, shticks) from the foundation Bowie and Ronson dreamed up together. And he does so without mentioning the words "heavy" and "metal" together in the same sentence. Which is strange when you consider that so much of what we now refer to as pop metal (i.e., Def Leppard, Poison, even early Mötley Crüe and Guns N’ Roses) was written from pages torn from Bowie’s Ziggy book, right on down to the spandex pants and back on up to the androgynous hairdos. But for some reason — and it’s one I can relate to — it would be heresy to blame heavy metal on Bowie instead of just giving him credit for "cool" music like punk and new wave and Marilyn Manson’s two best albums. It’s the same kind of thinking that makes it easier for me to appreciate AC/DC and Aerosmith as hard rock. And it’s why it’s going to take an awfully smart computer ever to do the simple task of telling one style of music from another.