Biology is the most extensive field in the Corpus Aristotelicum. In his fundamental work De anima, Aristotle tries to fix the borders of this life science. The term ψυχή has a twofold explanatory status. On the one hand, ψυχή is understood as a principle of all living beings. On the other hand, it is understood as a cause of the fact that all living beings are alive. The paper is divided into three sections. The first part shows why Aristotle discusses (...) these issues in a work entitled Περὶ ψυχῆς. Since Pythagoras and Heraclitus, ψυχή was understood as a life principle: Pythagoras believed that men, animals and plants share the same nature: they are all ἔμψυχα and they are homogenous qua ψυχή. The second part of this article deals with Aristotle’s definition of the soul in DA II: ψυχή is the principle of all living things. This establishes the external criteria to divide living and non-living beings and the internal criteria to divide living beings. The third part of this paper is concerned with the methodological consequences of this definition: the life functions are the central explanandum in Aristotle’s biology. De anima II defines such various life-functions as nourishment, sense-perception and locomotion. These capacities contour the main fields of the philosopher’s biological investigation. For Aristotle, the faculty of reproduction is a subtype of nourishment. Reproduction is the most important and most natural function of all living beings. Genetics is, therefore, the most important field in Aristotle’s biology. (shrink)