Imagine a world in which most of the knowledge you possess originates from the verbal reports of other people. These reports are usually based on the first-hand observations of the reporter, him- or herself, and the reporter's primary intention is to inform you of his or her observations. Now imagine another world descended from the first. In this second world, most of your knowledge still depends on others' reports, only now others' reports also come in the form of encoded symbols inscribed on relatively stable physical media. As in the first world, the primary intention of reporters is an informative one. The use of physical media in conjunction with verbal reports facilitates longer chains of reports: people often report to you another person's observations, the third person having inscribed his or her first-hand observation on a physical medium