Pneuma

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.

Pneuma's most consequential later transformation came through the Greek New Testament and Septuagint, where it renders the Hebrew ruach and becomes the standard word for spirit, including the Holy Spirit, Pneuma Hagion, carrying the term into Christian theology with a sense far more immaterial and personal than anything the Stoics intended; this is a genuine shift in meaning, not a straightforward continuation, since Christian pneuma names a divine person while Stoic pneuma names a physical medium. The Latin spiritus became the standard translation of pneuma in both the Stoic and Christian senses when Greek philosophy and scripture moved into the Latin-speaking world, a double duty that has caused persistent confusion between the physical Stoic concept and the theological Christian one ever since; the two are treated as separate entries in this atlas for exactly that reason.

Modern scholarship, surveyed in reference works such as the Stanford Encyclopedia of Philosophy's treatment of Stoicism, is careful to keep the Stoic physical sense distinct from later religious usage, since collapsing them anachronistically misreads Stoic materialism as a form of spiritualism it explicitly was not. As a concept internal to ancient philosophy and medicine, pneuma was never subjected to anything resembling a controlled modern test, and it is described here in the terms its own sources use.