"The living world is a sort of combinative place associating a finite number of elements and resembles the creation of a gigantic Mechanic. This is a totally changed perspective having come to light in the world of biology during the last few years.The scientist navigates between two poles : the desirable and the possible. Without the possibbl, the desirable is only a dream. Without the desirable, the possible is nothing more than boredom. It is often difficult to resist dreaming, but experimentation permits us to contain the imagination. At each stage, the scientist is obliged to expose himself to criticism and to experience in order to limit the dream-content in the work he elaborates. The scientific process is characterized by the non-stop confrontation of all that could be with all that is. Here, the author speaks of molecules, of reproduction and of the handywork of evolution. He also discusses the functioning of biologists, their way of viewing beauty and truth, good and evil." (F.J.)François Jacob was awarded the Nobel Prize in Medicine and has just been elected to the Académie Française.