Primitive man attributed the onset of serious disease as the influence of the gods, or possession by evil spirits. Over time, however, it was found that such "possessions" could be treated through the use of elixirs made from plant extracts. Methods were also developed to clean and treat wounds, and set fractured bones. The ancient Greeks established the first schools to teach medical sciences circa 500 BC. The texts produced by the students of these schools, most notably Hippocrates, who is considered the father of medicine, based his theories of the human body on observation and reasoning rather than supernatural intervention. Continued study of diseases over the years, combined with the dissection and study of the human body in the 13th century, pointed out flaws in early Greek theories, and led to more accurate medical texts based on detailed analysis rather than conjecture. By the 18th century, the same methods of analysis and observation used by scientists in other fields were applied to the study of medicine. This paved the way for the increasingly effective methods of treatment and surgery that have prevailed over the last 200 years.