The principle of life: from aristotelian psyche to drieschian entelechyIs life a simple result of a conjunction of physico-chemical processes? Can be reduce to a mere juxtaposition of spatially determined events? What epistemology or world-view allows us to comprehend it? Aristotle built a novel philosophical system in which nature is a dynamical totality, which is in constant movement. Life is its manifestation, and is formed and governed by the psyche. Psyche is the organizational principle of the different biological levels: nutritive, perceptive and intellective. Driesch's crucial experiment provided empirical proof of the principle of life, which he called 'entelechy'. Entelechy is an intensive manifoldness and cannot be comprehended by the usual extensive parameters. The entelechian's own ambiance is duration. This allows the reintroduction of the concept of teleology in the sphere of the living, understood not as a final cause, but as an order born from desire and leading to action