Gustav Fechner
Fechner, along with Wilhelm Wundt and Hermann von Helmholtz, is recognized as one of the founders of modern experimental psychology. His clearest contribution was the demonstration that because the mind was susceptible to measurement and mathematical treatment, psychology had the potential to become a quantified science. Theorists such as Immanuel Kant had long stated that this was impossible, and that therefore, a science of psychology was also impossible.
Menstrual psychosis
Founder of modern psychology, Emil Kraepelin, included an entry for "menstrual psychosis" in his 1909-1915 encyclopedia "Psychiatrie". This encyclopedia has been a major influence on the categories of the DSM and ICD. Kraepelin believed menstrual psychosis to have a hormonal cause.