The paper is the revised version of an inaugural lecture given as Lecturer ("Privatdozent") at the University of Zurich on 3 june 1985. It deals with the beginning and the main properties of “the science of nature” (hê peri physeôs historiê) (Plato.Phd.96a). According to Themistius (DK 12 A 7), the founder of this kind of Ionic philosophy is Anaximander of Miletus because he was the first who wrote about nature (especially a cosmography and a cosmogony) and developed three main principles of nature: (1) Nature has a mathematical structure (Arist. De coelo I3 295b10-14.32); (2) nature has a physical structure (DK 12 A 10-11); and (3) nature follows natural laws (DK 12 A 9). The origin of this introduction lies in the prolongation of primitive expectations of constancy. It is especially argued that the shift from mythos to logos is characterized by the depersonalization of nature and the introduction of proto-theoretical terms. In fact, the “apeiron” of Anaximander seems to be something similar to a theoretical term, because Anaximander seems to have realized that the explanans of nature cannot be one of its explananda, like water (Thales) or air (Anaximenes).