With our blogs and tweets, digital cameras, and unlimited-gigabyte e-mail archives, participation in the online culture now means creating a trail of always present, ever searchable, unforgetting external memories that only grows as one ages.

Growing up in the days when you still had to punch buttons to make a telephone call, I could recall the numbers of all my close friends and family. Today, I'm not sure if I know more than four phone numbers by heart. And that's probably more than most.

We're all just a bundle of habits shaped by our memories. And to the extent that we control our lives, we do so by gradually altering those habits, which is to say the networks of our memory. No lasting joke, or invention, or insight, or work of art was ever produced by an external memory. Not yet, at least.

Once upon a time, this idea of having a trained, disciplined, cultivated memory was not nearly so alien as it would seem to us to be today.

The best memorizers in the world - who almost all hail from Europe - can memorize a pack of cards in less than a minute. A few have begun to approach the 30-second mark, considered the 'four-minute mile of memory.'

During the Middle Ages they understood that words accompanied by imagery are much more memorable. By making the margins of a book colorful and beautiful, illuminations help make the text unforgettable. It's unfortunate that we've lost the art of illumination.

Evolution has programmed our brains to find two things particularly interesting, and therefore memorable: jokes and sex - and especially, it seems, jokes about sex.