The life force.
|Henri Bergson, French Philosopher|
Nevertheless it has an interesting place in the history of literature, turning up as it does in Romantic and Renaissance literature and onwards, only 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.
|Star Wars - Empire Strikes Back|
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).
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 scientific evidence for it's existence.