The idea of a vital soul being unnecessary to the existence or vitallity of those things. Each higher form possesses in full the attributes of the lower souls so that the human is the sole possessor of the rational soul. Thus the heart was the seat of the rational soul. The brain was seen as the vessel for distilling the pneuma. Galen summed up all the ideas then current, building up a paradigm which incorporated the atoms of Pythagorus, the four elements of Empedocles, the perfectly spherical divine universe of Plato, the 'primum mobile', the vegetable, animal and rational souls, and the quintessence of Aristotle, the four humours of Empedocles and Hippocrates and Erasistratus' notion that nerves are hollow tubes which carry quintessence from the brain, and added his own discovery of blood in the arteries with the contention that arterial blood was intermixed with quintessence.
Food is converted to natural spirits in the liver.
He noted that if the substance of the brain was cut at a given point, the animal did not lose consciousness or movement. For this to happen, the section had to penetrate as far as the ventricles. A lesion of the posterior ventricle had the most disturbing effect on the animal. For Plato and Galen the rational soul had its seat in the brain. The rational soul had the functions of imagination, reason and memory, and these functions were assigned to the ventricles.