Hans Eysenck (1916–1997) was one of the first psychologists to analyze personality differences using a psycho-statistical method (factor analysis), and his research led him to believe that temperament is biologically based. In his book "Dimensions of Personality" (1947) he paired Extraversion (E), which was "the tendency to enjoy positive events", especially social ones, with Neuroticism (N), which was the tendency to experience negative emotions. By pairing the two dimensions, Eysenck noted how the results were similar to the four ancient temperaments.