The Stoics built an entire physics around pneuma, describing it as a blend of the two active elements, fire and air, permeating the two passive elements, earth and water, and holding every object together through varying degrees of internal tension, tonos. The degree of pneumatic tension in a thing was said to determine its place in a graded hierarchy: mere hexis (cohesion) in inanimate objects such as stones, physis (growth) in plants, psyche (soul, sensation and impulse) in animals, and logos (reason) in humans and, at the summit, in the cosmos as a whole, which the Stoics held to be a single living, rational being.
Because Stoic philosophy was strictly materialist, holding that only body can act on body, pneuma was never an immaterial spirit in their system but a very fine, active kind of matter, the physical vehicle through which the cosmos's providential reason was literally, not metaphorically, present in things. Separately, the Greek medical tradition, from the Hippocratic writers through Galen in the second century CE, used pneuma for the vital air drawn in by breathing and refined in the body, with Galen distinguishing pneuma physikon (natural pneuma, in the liver, governing nutrition), pneuma zotikon (vital pneuma, in the heart, governing life-heat) and pneuma psychikon (animal or psychic pneuma, in the brain, governing sensation and movement), a scheme that dominated Western medical physiology until the seventeenth century.