Henri Bergson, answered from the texts and cited to the page.
The élan vital is not a substance, not a fluid in the cell, not a force in the physicist's sense. It is the image I condensed from a cluster of ideas that pure experience of life forces upon any honest observer — ideas which, taken together, give the concept its content and save it from the emptiness of a phrase like "the will to live." What are those ideas?1
Life proceeds not as a single line but as a diverging fan of lines, some running freely forward, others arriving at dead ends.1 At the extremities of the two principal animal lines we find instinct and intelligence — tendencies so different that one might take them for opposed natures, yet they must be understood as having been one in the beginning, not combined but undivided, separating only as the lines diverged.1
And running through all of this is the impossibility — which neither mechanism nor finalism can honestly face — of forecasting in advance the forms life will create. Pure mechanism supposes the future is calculable from the present; pure finalism supposes it is already designed within it as an idea. In either case, time is unavailing, a mere backdrop.1
Pure experience suggests nothing of the sort. The élan vital carries, in its very image, the sense of a creative time in which genuinely new forms emerge — not realizations of pre-existing possibilities, but inventions. Yet this force is limited. It is always seeking to transcend itself and always remains inadequate to the work it would produce.2
Most often the effort turns short — paralyzed by contrary forces, diverted from what it should do by what it does, absorbed by the form it is engaged in taking, hypnotized by it as by a mirror.2 Our own inner life shows this directly: freedom, in the very movements by which it is affirmed, creates the growing habits that will stifle it if it fails to renew itself by a constant effort.2
The most living thought becomes frigid in the formula that expresses it. The word turns against the idea.2 This is not a defect peculiar to human consciousness; it is the signature of the vital impulse throughout the organized world, where the disproportion between the work and the result is everywhere striking. What the élan vital names, in the domain of biological evolution, is the inward impulse that passes from germ to germ through individuals, carrying life in a given direction toward ever higher complexity — not the mechanical action of external causes pressing a mould upon clay, but the original solution that life finds for the problem set by external conditions.3
The faculty of finding such solutions is what mechanism systematically denies in its general theory while quietly invoking in every particular case.3 I did not claim to explain that faculty; I brought its mysterious character into the open rather than concealing it behind a false mechanistic clarity.3 The vital impulse is transmitted through generations of organisms, and it is what the nervous system — a veritable reservoir of indetermination, with manifold ways opening at every junction — expresses most fully.2
The energy spent in creating such apparatus is, I believe, what a glance over the organized world as a whole most readily reveals.2 Indetermination is not mere absence; it is the condition for genuine creation, for the response that is not mechanically fixed in advance. Neglect these ideas, and the élan vital becomes an empty concept.1 Take them into account, and it becomes an idea full of matter, obtained empirically, capable of guiding investigation — one that broadly sums up what we know of the vital process and brings out what remains unknown.1
Such were the ideas we condensed into the image of the 'vital impetus.' To neglect them, as has been too often done, is to find oneself confronted by an empty concept, like that of the pure 'will to live,' and by a barren theory of metaphysics. By taking them into account, we have an idea full of matter, obtained empirically, capable of guiding our investigations.The Two Sources of Morality and Religion, p. 111
The force which is evolving throughout the organized world is a limited force, which is always seeking to transcend itself and always remains inadequate to the work it would fain produce... Even in its most perfect works, though it seems to have triumphed over external resistances and also over its own, it is at the mercy of the materiality which it has had to assume.Creative Evolution, pp. 170–171
it is not the mechanical action of external causes, but an inward impulse that passes from germ to germ through individuals, that carries life in a given direction, towards an ever higher complexity... instead of systematically rejecting it in general while resorting to it on the sly in each particular case, we brought out this mysterious character of the operation of life.The Two Sources of Morality and Religion, p. 109