He emphasized the role of pneuma, ('breath' or 'spirit') in the functioning of the soul; soul-activities were explained by pneuma extending throughout the body from the 'ruling part' located in the head. All sensation is felt in the ruling-part of the soul, rather than in the extremities of the body; all sensation involves thought, and there is no thought not derived from sensation. He denied that the soul was immortal, and attacked the 'proofs' put forward by Plato in his "Phaedo".