"Bergson introduced the idea of élan vital because he became convinced that Darwinism was a dead-end. Schematically his argument is as follows. Darwinism is the strongest version of biological mechanism since Democritus; therefore, the refutation of Darwinism delivers the coup de grâce to mechanical biology. As for the refutation, in thinking about evolution we must not forget time's irreversible, anisotropic quality, which as an event or process biological evolution inherits and which excludes mechanical causation. Mechanical causation is reversible, life and evolution are not, and nothing irreversible can be the effect of a reversible cause. With biological mechanism a dead letter, Bergson unfolds the rest of his argument. The evolution of terrestrial life is a single movement, understanding movement as an indivisible, qualitatively singular duration. This movement began some four billion years ago and remains ongoing, a movement that has yet to reach its end. This movement is vital because it begins with the origin of terrestrial life and has evolved to the present, and Bergson describes it as an élan because this ancient and still vital event has the impetuous, tendentious quality of a tendency, striving to become all that it can be. The originating impetus of the event that is life "is finite and it has been given once for all." (CE 254) Life at the origin already has all the tendency ever to be expressed in terrestrial evolution"--