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A few months ago my Tube pass started playing up, and after a day of dark looks as I took two, three or even four goes to get through the barriers, I took it to a ticket officer. He touched it to his card reader and told me something surprising. “On January 17 you got on the Tube at Leicester Square,” he said, without the hint of a smile. “According to the computer, you never got off.”

I couldn’t remember where I’d been on that winter’s day six months before, but I’ve not forgotten since that every journey made on London’s transport network can be tracked and catalogued by the microchip inside my travelcard. Even so, I’ve been happy to keep using it, laying an electronic trail as I go.

And that is a neat illustration of why privacy is, in theory, the biggest question of the information age and, in practice, something that barely troubles us. We may all say that we value our privacy, but given the chance to save a little bit of time or money, we willingly give it up.

Privacy is dead. Long live privacy

Jeffrey Rosen: Security doesn't have to encroach on privacy. Technology can help to protect both

The journeys we make, the products we buy, the websites we visit, the e-mails we write – all are logged, indexed and stored, sometimes for years. Soon medical records and biometric data will flesh out these details, and many of us put a human face on all the facts and figures by posting photographs on Facebook and Flickr.

The problem for privacy campaigners is that the risks seem rather nebulous when set against the practical advantages of all these technologies. I could keep my journeys to myself by using a paper ticket instead of a swipe card, but then I wouldn’t be able to renew it online. So I brush aside my half-formed fears of Big Brother for the sake of avoiding a fully formed queue.

From time to time the risks become more tangible. Last year AOL released some interesting and alarming examples of what its customers had been searching for, along with data allowing many of them to be identified, and British junior doctors recently found that details of their religion and sexuality had been released onto the web.

The leaks provoked some short-lived fury, but were treated as isolated incidents rather than symptoms of an online culture in which personal data can be accumulated so easily and stored so casually. That’s not surprising. None of us wants to think too hard about the data we leave behind us, and until a truly enormous breach causes real hassle or embarrassment, we all have more pressing concerns. Avoiding ticket queues, for example.

Click here to read Jeffrey Rosen's response: Privacy is dead. Long live privacy

A Battle of Ideas debate on "Privacy is dead. Long live privacy?" will take place on Saturday, October 27 at 15.30

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Holden Frith is Tech & Web Editor of Times Online