It's absolutely everywhere. From on-line bookstores, to targetted mailshots, the concept is simple. If you bought product X, you'll want to buy product Y.

	The Beastie Boys, Adiemus, Frank Sinatra, Harvey Danger and the Dave Matthews Band. I don't actually care for any of these.	Hello, James Gleick," said the Amazon.com Web site the other day. (Click here if you're someone else.) "Take a peek at your brand-new music recommendations." I peeked. Amazon's computers predicted that I would like the Beastie Boys, Adiemus, Frank Sinatra, Harvey Danger and the Dave Matthews Band. What an impressive list! All right, I don't actually care for any of these. But still, it was quite a shot in the dark, considering I'd never been to Amazon's music department before. This is the way it's going on the Internet: if marketers want your money and your time and your "eyeballs," they feel they should figure out who you are and what you like. Not only does their software try to calculate your taste in music by keeping track of the music you buy, it even tries to work out your taste in music from your reading habits. This could be a parlor game: If you like Vladimir Nabokov, maybe you'll also like Igor Stravinsky? If you like "War and Peace," maybe you'll like the "1812 Overture"? If you like E.L. Doctorow's "Ragtime," maybe you'll like Scott Joplin's ragtime? "One Flew Over the Cuckoo's Nest" and 10,000 Maniacs? "Consumer Reports" and Crash Test Dummies? Kafka's "Metamorphosis" and the Beatles? We like to believe that our souls are our own and there's no accounting for taste. So it's disconcerting to find that, on line, there's suddenly lots of accounting for taste. Amazon has its Bookmatcher; the music store CD Now has its Album Advisor -- sooner or later every merchant of just about everything will follow suit, analyzing your private likes and dislikes with "real-time recommendation engines" based on "collaborative filters" and fuzzy logic. The basic idea is the same everywhere. Say you favor turtlenecks, convertibles, nautical history, bebop and zinfandel. No doubt you are proud of your rare good taste, but it's a big world, and somewhere in a million-entry electronic-commerce database are a few other people with the same preferences.
	The whole thing is just a database look-up, but maybe it works for some people and some kinds of taste.	If you knew that your doppelgangers were raving about the latest Mike Leigh movie, wouldn't you want to give it a look? In the jargon of the collaborative-filter game, these weird pals are your "community" and your "trusted associates." Their taste might be more in tune with yours than the few people you trust in your own small circle of friends. The whole thing is just a mildly clever database look-up, but maybe it works, at least for some people and some kinds of taste. It has no intelligence about the content of the merchandise -- Mozart and Madonna might be flavors of ice cream, for all it knows. It only has the beginnings of what could become a formidable electronic dossier: your purchasing history plus your volunteered comments about what you love and what you hate. At CD Now, for example, a customer can choose buttons for "I own this already" or "Not for me." The computers, of course, watch and learn. It's scary. "Is one's entire psyche's most secret landscape really a fairly public thing, given just a few pieces of information?" asks Douglas R. Hofstadter, the cognitive theorist and author of "Godel, Escher, Bach." "If you know that I love Chopin and Bach and am totally cool to Beethoven, can you predict that I love Cole Porter and Fats Waller but am indifferent to Oscar Peterson and Charlie Parker, and hate Elvis Presley? "What is disturbing, to spell it out, is the idea that one's taste, which seems like such a personal thing, connected with and determined by one's inmost being, should have, in a way, a mechanically, nearly deterministically, knowable nature." Maybe we're not quite knowable at that. And these are computers, so the mistakes they make can look very, very stupid. When they go off the rails in a sensitive area like taste, some people get angry. "The worst thing of all," says one irate customer, Russ Korins, "if you like Third Eye Blind -- who sing 'Semi-Charmed Life,' that song that goes doo doo doooo, doo doo DOO do, and 'Graduate,' a mainstream version of Avenue A rebel rock, screaming, 'Can I graduate?!' -- then what do they also recommend? 'The Four Seasons' by A. Vivaldi."
	If you like Third Eye Blind then what do they also recommend? The Four Seasons by A. Vivaldi.	At CD Now, they take this sort of thing in stride. "You can't argue with the customers -- they know what they like," says Evan Schwartz, director of product management. "People love to click on 'Not for me,' 'Not for me."' When customers click, and especially when they buy, they add to the storehouse of information about that mercurial, irrational, chaotic thing we call taste. As knowledge builds up, maybe the computers will stop recommending Vivaldi to Third Eye Blind fans. Or maybe it will turn out that Vivaldi and Third Eye Blind have some kind of century-bridging affinity, even if no musicologist could say exactly what. Or maybe it's a moving target, and last year's Third Eye Blind fans have a different sensibility from this year's. You might think of these growing databases as merchandising dossiers. Marketers are keeping a file on you, and if it's not as tangible or incriminating as your F.B.I. file, it's too personal for comfort. "These exhaustive lists become much more than mere lists; they act as electronic psychoanalysts," writes David Shenk in his 1997 book, "Data Smog." Under pressure from privacy advocates, most companies in the collaborative-filtering business pledge not to share information without a customer's consent. Even if the trail of your reading history leads your bookseller to conclude that you're on the verge of buying a new red Porsche Boxster or a blue Gap dress, Amazon promises not to tell the car companies or the special prosecutor. Still, if we have learned anything, we know that information tends to get around. Do I really want the whole Web to know I'm a Beastie Boys, Frank Sinatra, Harvey Danger kind of guy? I didn't even know that myself.

As research for this article, I asked Amazon what my preferences were, based on previous orders for albums from the Barenaked Ladies, Jewel, Alanis, Faith Hill, the Now compilation and the Rugrats movie soundtrack. They hit with the new BNL album, and came reasonably with Lauryn Hill and the Dixie Chicks. But no mention of Brittany Spears, or Steps.