The classic treatment of crowds is Gustave LeBon, "The Crowd: A Study of the Popular Mind" , in which the author interpreted the crowds of the French Revolution as irrational reversions to animal emotion, and inferred from this that such reversion is characteristic of crowds in general. LeBon believed that crowds somehow induced people to lose their ability to think rationally and to somehow recover this ability once they had left the crowd. He speculated, but could not explain how this might occur. Freud expressed a similar view in "Group Psychology and the Analysis of the Ego" (1922). Such authors have thought that their ideas were confirmed by various kinds of crowds, one of these being the economic bubble. In Holland, during the tulip mania (1637), the prices of tulip bulbs rose to astronomical heights. An array of such crazes and other historical oddities is narrated in Charles MacKay's "Extraordinary Popular Delusions and the Madness of Crowds" .