Spiritus

Latin Stoicism, represented by writers such as Seneca, Cicero (reporting Stoic doctrine even where he argued against it) and later Marcus Aurelius in Greek, inherited the Greek Stoic concept of pneuma and rendered it as spiritus, keeping the same physics: a fine blend of fire and air pervading all matter, holding bodies together through internal tension and grading upward from mere cohesion in stones to reason in human beings and in the cosmos as a whole. Seneca's Natural Questions and moral letters use spiritus in ways that keep this Greek technical sense largely intact, treating it as the active, material principle animating the world rather than as an immaterial soul distinct from body, consistent with Stoic materialism generally.

This philosophical spiritus predates the Christian use of the same word by centuries and belongs to a different conceptual system entirely, even though later readers, encountering both uses in the same language, did not always keep them apart.

Renaissance Neoplatonists extended spiritus further still: Marsilio Ficino, in his Three Books on Life (1489), described spiritus as a thin, warm vapor bridging body and soul, the medium through which the influence of the stars, planets and even talismans was believed to act on a person, folding the older Stoic physics into Renaissance astrological medicine and natural magic. This spiritus mundi, world-spirit, was treated by Ficino and later Hermetic writers as continuous with an individual's own spiritus, so that a person attuned through diet, music or astrologically timed talismans could draw down and strengthen the more vital, celestial kind of spirit, a use further removed still from the Christian theological sense of Spiritus Sanctus described in the article below.

When Jerome translated the Hebrew Bible and Greek New Testament into Latin as the Vulgate in the late fourth century, he rendered both the Hebrew ruach and the Greek pneuma as spiritus, fixing the word as the standard Latin term for spirit in Western Christianity, including Spiritus Sanctus, the Holy Spirit, the third person of the Trinity in Christian doctrine. This theological spiritus is personal and immaterial in a way Stoic spiritus, strictly speaking, is not, since Christian theology holds the Holy Spirit to be a divine person rather than a subtle physical substance, even where certain early Christian writers, working in a culture still shaped by Stoic physics, described spirit in language that echoes the older materialist vocabulary.

The result is a single Latin word carrying two genuinely different doctrines forward together, Stoic physical spiritus and Christian theological Spiritus Sanctus, a conflation that later Latin readers, including many medieval and Renaissance writers on the "animal spirits" of physiology, inherited and did not always disentangle; those spirits of early modern medicine, discussed in figures from Galen's Latin translators through to Descartes, are a further, still more diluted descendant of the same word. As with pneuma, this is a philosophical and theological history rather than an experimental claim, and no scientific test of spiritus as a substance has ever been proposed within either tradition.