Bergson regarded planning beforehand for the future as impossible, since time itself unravels unforeseen possibilities. Indeed, one could always explain a historical event retrospectively by its conditions of possibility. But, in the introduction to the "Pensée et le mouvant", he explains that such an event created retrospectively its causes, taking the example of the creation of a work of art, for example a symphony: it was impossible to predict what would be the symphony of the future, as if the musician knew what symphony would be the best for his time, he would realize it. In his words, the effect created its cause. Henceforth, he attempted to find a third way between mechanism and finalism, through the notion of an original impulse, the "élan vital", in life, which dispersed itself through evolution into contradictory tendencies (he substituted to the finalist notion of a teleological aim a notion of an original impulse).