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.