Final answer:
Gall assumed that specific mental functions were tied to distinct regions of the brain, which could be assessed through skull shape and size, known as phrenology.
Step-by-step explanation:
Franz Gall, a German physician, was a proponent of phrenology, a now discredited field that linked skull shape and size to personality and mental faculties. Gall assumed that mental functions were localized in specific 'mental organs,' with each area of the brain correlating to different intellectual abilities or personality traits.
Through the analysis of the shape and size of the skull, Gall believed he could infer the strength and development of the underlying brain areas and thus a person's capabilities and character.