The great philosopher and historian of philosophy, Étienne Gilson, sets out to show that final causality or purposiveness and formal causality are principles for those who think hard and carefully about the world, including the world of biology. Gilson insists that a completely rational understanding of organisms and biological systems requires the philosophical notion of teleology, the idea that certain kinds of things exist and have ends or purposes the fulfillment of which are linked to their natures--in other words, formal and final causes. His approach relies on philosophical reflection on the facts of science, not upon theology or an appeal to religious authorities such as the Church or the Bible.