Full context:   Modern medical science has rejected the theories of the four temperaments, though their use persists as a metaphor within certain psychological fields. Immanuel Kant (1724–1804), Erich Adickes (1866–1925), Alfred Adler (1879–1937), Eduard Spranger (1914), Ernst Kretschmer (1920), and Erich Fromm (1947) all theorised on the four temperaments (with different names) and greatly shaped modern theories of temperament. Hans Eysenck (1916–1997) was one of the first psychologists to analyse personality differences using a psycho-statistical method called factor analysis, and his research led him to believe that temperament is biologically based. The factors that he proposed in his book Dimensions of Personality were neuroticism (N), the tendency to experience negative emotions, and extraversion (E), the tendency to enjoy positive events, especially social ones. By pairing the two dimensions, Eysenck noted how the results were similar to the four ancient temperaments.