Emil Kraepelin (1855-1925) was a psychiatrist who studied the description and Classification of mental disorders, leading to what we now call the Diagnostic and Statistical Manual of Mental Disorders, published by the American Psychiatric Association.
He believed that if symptom clusters could be identified then research could more easily begin the work of identifying the etiology or causation. He is likely best known for identifying and naming dementia praecox, the precursor to what we now call schizophrenia.