Following ideas inspired by Vannevar Bush's famous 1945 article "As We May Think", Garfield undertook the development of a comprehensive citation index showing the propagation of scientific thinking; he started the Institute for Scientific Information in 1955. The creation of the Science Citation Index made it possible to calculate impact factor,[4] which measures the importance of scientific journals. It led to the unexpected discovery that a few journals like Nature and Science were core for all of hard science. The same pattern does not happen with the humanities or the social sciences.