Henri Bergson, answered from the texts and cited to the page.
The élan vital is not a mysterious fluid secreted by living matter, not a vitalist substance posited to fill the gaps left by chemistry. It is an image — one earned empirically, not stipulated — for the differentiating tendency by which life pushes forward, splitting at each turn into divergent lines, creating forms that no calculation from prior conditions could have predicted.1
Consider what evolution actually shows us. Along two principal lines — the arthropods culminating in insect instinct, the vertebrates culminating in human intelligence — a single original impulsion has distributed itself, separating tendencies that were at first wrapped within one another.2 Instinct and intelligence are not two different things that happened to evolve side by side; they are two views taken from different angles of one simple reality, one common vital movement that divided as it met resistance in matter.3
The force bore both within it, confused together, the one implied in the other — and it is the fact of their divergence that lets us conjecture what the source must have been. That source is limited. This is what radical finalism always misses: it imagines the whole of the living world as a construction perfectly adequate to its design, each species with its allotted part in a fundamental harmony.4
But look at what evolution actually produces — effort turning short, paralyzed by contrary forces, absorbed by the very form it is engaged in taking, hypnotized by it as by a mirror.5 The élan vital is a force always seeking to transcend itself and always remaining inadequate to the work it would fain produce. That inadequacy is not a refutation of the idea; it is part of what the idea names.
What the image most essentially conveys is the impossibility of forecasting, in their entirety, the forms life creates. Neither mechanism nor finalism can account for genuine novelty: both suppose the future deducible from the present, whether by calculation or by pre-given design, rendering time unavailing.6 Pure experience suggests nothing of the sort.
The élan vital is precisely what "neither impulsion nor attraction" points toward — a creative movement whose results could not have been read off from anything that preceded them, and whose real duration is the essential attribute of life itself. The highest expression of this movement is not the artist, whose beautiful forms indicate an expansion of life but also a momentary stop of its impulse, a rounding-off rather than a pressing-forward.7
It is the moral man — those figures of inventive and simple heroism who have opened new paths to virtue — in whom the vital movement pursues its way without hindrance. They are nearest the source, and it is by studying such lives, by striving to experience sympathetically what they experience, that one may penetrate by an act of intuition to the life principle itself.8
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
On two lines only has it achieved an undeniable success, partial in the one case, relatively complete in the other. These are the lines of evolution of the arthropods and the vertebrates. At the end of the first line, we find the instincts of the insect; at the end of the second, human intelligence.I. Life and Consciousness, pp. 32–33
instinct and intelligence, which reach their culminating point at the extremities of the two principal lines of animal evolution, must therefore be taken one with the other, before their separation: not combined into one, but one in the beginning, instinct and intelligence being then mere views, taken from two different points, of that simple reality.The Two Sources of Morality and Religion, p. 111
It has represented the whole of the living world as a construction, and a construction analogous to a human work. All the pieces have been arranged with a view to the best possible functioning of the machine. Each species has its reason for existence, its part to play, its allotted place; and all join together, as it were, in a musical concert, wherein the seeming discords are meant to bring out a fundamental harmony.Creative Evolution, pp. 170–171
There, the disproportion is striking between the work and the result. From the bottom to the top of the organized world we do indeed find one great effort; but most often this effort turns short, sometimes paralyzed by contrary forces, sometimes 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.Creative Evolution, pp. 170–171
Whether you embrace the doctrine of pure mechanism or that of pure finality, in either case the creations of life are supposed to be predetermined, the future being deducible from the present by a calculation, or designed within it as an idea, time being thus unavailing. Pure experience suggests nothing of the sort.The Two Sources of Morality and Religion, p. 111
Richness and originality of forms do indeed indicate an expansion of life, but in this expansion, where beauty means power, life also shows a stop of its impulse, a momentary powerlessness to push farther, like the boy who rounds off in a graceful curve the end of the slide.I. Life and Consciousness, p. 40
The men of moral grandeur, particularly those whose inventive and simple heroism has opened new paths to virtue, are revealers of metaphysical truth. Although they are the culminating point of evolution, yet they are nearest the source and they enable us to perceive the impulsion which comes from the deep. It is in studying these great lives, in striving to experience sympathetically what they experience, that we may penetrate by an act of intuition to the life principle itself.I. Life and Consciousness, p. 40