  Knowledge is useless without structure. While the classification of knowledge
has been an enduring philosophical enterprise, it recently found applications
in computer science, notably for artificial intelligence. The availability of
large databases allowed for complex ontologies to be built automatically, for
example by extracting structured content from Wikipedia. However, this approach
is subject to manual categorization decisions made by online editors. Here we
show that an implicit classification hierarchy emerges spontaneously on
Wikipedia. We study the network of first links between articles, and find that
it centers on a core cycle involving concepts of fundamental classifying
importance. We argue that this structure is rooted in cultural history. For
European languages, articles like Philosophy and Science are central, whereas
Human and Earth dominate for East Asian languages. This reflects the
differences between ancient Greek thought and Chinese tradition. Our results
reveal the powerful influence of culture on the intrinsic architecture of
complex data sets.
