The principles upon which phrenology was based were five: (1) the brain is the organ of the mind; (2) human mental powers can be analyzed into a definite number of independent faculties; (3) these faculties are innate, and each has its seat in a definite region of the surface of the brain; (4) the size of each such region is the measure of the degree to which the faculty seated in it forms a constituent element in the character of the individual; (5) the correspondence between the outer surface of the skull and the contour of the brain-surface beneath is sufficiently close to enable the observer to recognize the relative sizes of these several organs by the examination of the outer surface of the head.

The system of Gall was constructed by a method of pure empiricism, and his so-called organs were identified on quite specious grounds. Having arbitrarily selected the place of a faculty, he examined the heads of his friends and casts of persons with that peculiarity in common, and in them he sought for the distinctive feature of their characteristic trait. Some of his earlier studies were made among inmates of jails and lunatic asylums, and some of the traits that he presumed to detect were “criminal.” These he named after their excessive manifestations, mapping out organs of murder, theft, etc. However, the names were changed by Spurzheim to align with more moral and religious considerations. Gall marked out on his model of the head the places of 26 organs as round enclosures with vacant interspaces. Spurzheim and Combe divided the whole scalp into oblong and conterminous patches identified by such designations as amativeness, philoprogenitiveness, concentrativeness, adhesiveness, combativeness, destructiveness, secretiveness, acquisitiveness, constructiveness, self-esteem, love of approbation, cautiousness, benevolence, veneration, conscientiousness, firmness, hope, wonder, ideality, wit, imitativeness, individuality, form perception, size perception, weight perception, colour perception, locality perception, number perception, order perception, memory of things, time perception, tune perception, linguistic perception, comparative understanding, and metaphysical spirit.