Kissing could have begun as a way of sniffing out who's who. From a whiff to a kiss was just a short trip across the face.

One trick, known as the journey method or 'memory palace,' is to conjure up a familiar space in the mind's eye, and then populate it with images of whatever it is you want to remember.

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.

Some memorizers arbitrarily associate each playing card with a familiar person or object, so that the king of clubs is represented by, say, Tony Danza. The grand masters associate each card with a person, an action, or an object so that every group of three cards can be converted into a sentence.

We've outsourced our memories to digital devices, and the result is that we no longer trust our memories. We see every small forgotten thing as evidence that they're failing us.

If you were a medieval scholar reading a book, you knew that there was a reasonable likelihood you'd never see that particular text again, and so a high premium was placed on remembering what you read. You couldn't just pull a book off the shelf to consult it for a quote or an idea.

Just as we accumulate memories of facts by integrating them into a network, we accumulate life experiences by integrating them into a web of other chronological memories. The denser the web, the denser the experience of time.