In folk belief, spirit is the vital principle or animating essence within all living things. As recently as 1628 and 1633 respectively, both William Harvey and René Descartes still speculated that somewhere within the body, in a special locality, there was a "vital spirit" or "vital force", which animated the whole bodily frame, just as the engine in a factory moves the machinery in it. Overview People have frequently conceived of spirit as a supernatural being, or non-physical entity; for example, a demon, ghost, fairy, or angel. In ancient Islamic terminology however, the term spirit (rūḥ), applies only to "pure" spirits, but not to other invisible creatures, such as jinn, demons and angels. Historically, spirit has been used to refer to a "subtle" as opposed to "gross" material substance, as put forth in the notable last paragraph of Sir Isaac Newton's Principia Mathematica. In English Bibles, "the Spirit" (with a capital "S"), specifically denotes the Holy Spirit. The concepts of spirit and soul often overlap, and some systems propose that both survive bodily death. "Spirit" can have the sense of "ghost", i.e. a manifestation of the spirit of a deceased person. "Spirit" is also often used to refer to the consciousness or personality. Etymology The word spirit came into Middle English via Old French esperit. Its source is Latin spīritus, whose original meaning was "breath, breathing" and hence "spirit, soul, courage, vigor"; its ultimate origin is a Proto-Indo-European root . In Latin, was distinct from Latin anima, whose etymological meaning was also "breathing" (PIE root *h₂enh₁-), yet which had taken a slightly different meaning, namely "soul". Classical Greek also had a similar distinction between "soul" and "spirit", in each case involving again an etymological sense "breathing": (ψυχή), originally "cold air", hence "breath of life" and "soul" (PIE root "to breathe"). (πνεῦμα) "breath, motile air, spirit", from verb (πνέω) "to breathe". A distinction between soul and spirit also developed in the Abrahamic religions: Arabic (نفس) opposite (روح); Hebrew neshama ( ) or nephesh ( ) (in Hebrew comes from the root or "breath") opposite ( ). (Note, however, that in Semitic just as in Indo-European, this dichotomy has not always been as neat historically as it has come to be taken over a long period of development: Both (root ) and (root ), as well as cognate words in various Semitic languages, including Arabic, also preserve meanings involving miscellaneous air phenomena: