Dementia praecox is a term which Bénédict Augustin Morel first used in 1860, to describe the condition of a teenager who retreated from society, and started to show symptoms similar to those of dementia. Dementia is a mental illness which mostly occurs in old people.
Emil Kraepelin used the term to describe, and contrasted it with the mental illness called Bipolar disorder. People with bipolar disorder often have episodes in which they seem normal, which people with dementia praecox do not.
This theory could not be proved. In 1911, Eugen Bleuler said that dementia praecox was actually schizophrenia. The word Schizophrenia was not used widely before 1925.