Our culture is an edifice built of externalized memories.

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.

Our ability to find humor in the world, to make connections between previously unconnected notions, to create new ideas, to share in a common culture: All these essentially human acts depend on memory.

How much are we willing to lose from our already short lives by losing ourselves in our Blackberries, our iPhones, by not paying attention to the human being across from us who is talking with us, by being so lazy that we're not willing to process deeply?

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

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.

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.