(p. 17) Page, a child of academia, understood that web links were like citations in a scholarly article. It was widely recognized that you could identify which papers were really important without reading them– simply tally up how many other papers cited them in notes and bibliographies. Page believed that this principle could also work with web pages. But getting the right data would be difficult. Web pages made their outgoing links transparent: built into the code were easily identifiable markers for the destinations you could travel to with a mouse click from that page. But it wasn’t obvious at all what linked to a page. To find that out, you’d have to somehow collect a database of links that connected to some other page. Then you’d go backward.
Levy, Steven. In the Plex: How Google Thinks, Works, and Shapes Our Lives. New York: Simon & Schuster, 2011.