Percival Sylvester Bailey (May 9, 1892 – August 10, 1973) was an American neuropathologist, neurosurgeon and psychiatrist who was a native of rural southern Illinois.
In 1918 he graduated from Northwestern University in Evanston, and in 1919 became an assistant to Harvey Cushing at Peter Bent Brigham Hospital in Boston.
From 1922 to 1925, Bailey performed extensive pathological and histological studies of brain tumors, and based on cellular configuration, he created a classification system of thirteen categories.
With Gerhardt von Bonin (1890-1979), Bailey authored two works, "The Neocortex of the Chimpanzee" in 1950 and "The Isocortex of Man" in 1951, which provided an accurate description concerning the cytoarchitecture of the cerebral cortex.
As a psychiatrist, Bailey was a vocal critic of Freudian psychology, which he considered speculative and unscientific.