The life force
Bergson came up with this as a sort of driving force behind all life, that is unknown to natural science, but to be fair, he wasn't the first, or indeed the last, his name just got linked to it, and so he achieved a kind of grim immortality from it. Ironic, since it doesn't really carry much weight any more, mainly because of the success of modern biological, chemical, and physical sciences in explaining the processes and structures underlying life. Nevertheless it has an interesting place in the history of literature, turning up as it does in Romantic and Renaissance literature and onwards, to suffer a huge decline in the modern era, though some would say it's made a comeback recently in Magical Realism, and the realms of the fantastic including but not limited to The Force in Star Wars.
The notion that there is an underlying life force is a very appealing one, and can also be found in many religions and spiritual movements. There is a possible equivalent in the "Great Spirit" of the American Indians, as well as the the deity "Vishnu" in Hinduism, who as you'll recall is the preserver in the Hindu Trinity of Brahma (Creation), Vishnu (Preserver), and Shiva (Death and Destruction).
A Catholic/Islamic equivalent can be found personified in the Archangel Michael, supposedly the life sustaining Angel, with the equivalent Archangel Mika'il in Islam, forming one of a trio of Archangels present in both religions that cover the same philosophical duties of life. Namely Gabriel for its creation, Michael for its sustenance, and Azrael for its death. Jibra'il, Mika'il, and Isra'il being the Islamic equivalents.
The elan vital can also be seen as as currency for the soul here, it being bestowed, moved about through the economy of cosmological existence, and transferred to the afterlife by the Angel of Death. While its metaphorical and emotional appeal is considerable, there doesn't yet appear to be any shred of scientific evidence for it's existence.