Not far from the line of Heraclitus, Democritus, about 460 BC, extended the psychology of atomism of his predecessor Leucippus and taught that the human soul was a substance composed of very subtle and spherical atoms, as those of fire, and it was precisely because of their subtleness and sphericity that these atoms contribued to perpetual motion and heat of both the fire and the human soul.
The soul, to Democritus, is a very subtle and volatile element that exists within the body (made of much more inert and gross material). This volatile soul spreads and penetrates all parts of body tissues and produces the different organs and limbs own vital functions... The thought, consciousness and sensation, particularly, would be the result of a form of variable combination of these ethereal and spherical atoms. The unstable combinations of these would be the source of psychic manifestations and their fluctuations over time.
The atoms of the soul have a continuous circular motion which says Democritus is sustained by the inspiration and expiration of air. Breathing, therefore, is the essential process of life and of the psychic manifestations that occur over time. These are, again, breathing and air.
What the philosopher of Abdera called spirit is not a supreme creative force in the world, or anything supernatural or metaphysical, not even a principle of nature 'superior' to mechanical motion. It is not, in fact, an essentially different force from mechanical movement. What we call 'spirit', like 'soul', is just a more subtle material (air) compared to other grosser and solid (the body) or, if preferred, is the phenomenon resulting from the properties of these finer and subtler air atoms we breathe in their action on our body atoms.
Also, gods, whose existence he admits, are for Democritus basically the same: spirits, beings like the soul in its origin and composition, with no other difference with soul than having a more durable arrangement of their atoms, a longer life, but this is the only respect in which gods and spirits are superior to men and their soul. Gods are also mortal and subjected, like us, to the supreme law of fate, the only constant, ie the law of eternal, the necessary and universal motion of atoms, the unfailing flow of organization and disorganization of matter. Gods, spirits, soul, mind, consciousness, sensation, thought... are essentially the same: they are the similar phenomena resulting from the action of air in our body through breathing.
Cappelletti, A. J. Los fragmentos de Diógenes de Apolonia. Tiempo Nuevo, Caracas, 1975.
Cappelletti, A. J. Mitología y filosofía: los presocráticos. Cincel, Madrid, 1987.
Cicerón, M. T. Sobre la naturaleza de los dioses. UNAM, México, 1986.
Fernández Cepedal, J. M. Los filósofos presocráticos. Proyecto Filosofía en español, www.filosofia.org.
Conde, F. Filósofos presocráticos. Página sobre filosofía, www.paginasobrefilosofia.com.
González, C. Historia de la filosofía. 2 ª ed, Madrid, 1886. Edición digital Proyecto Filosofía en español, www.filosofia.org.