This article is the first one to offer an investigation, from a biological perspective, of “natural philia” or “kin-based” philia in Aristotle’s practical philosophy. After some preliminary considerations about its place in Aristotle’s ethical treatises, the discussion focuses on Aristotle’s biology. Here we learn that natural philia, couched in terms of a biological praxis rather than a trait of character, is widespread in the animal kingdom, although in different ways and to varying degrees. To account for such differences, Aristotle establishes (...) a Scala Philiae in two different biological texts—Historia Animalium and Generation of Animals—where natural bonds in animals are classified in view of their strength and duration. Each level of Aristotle’s Scala is examined. Finally, the argument returns to Aristotle’s ethical and political texts, drawing greater attention to the biological mechanisms that underlie natural philia in human beings. I conclude that natural philia provides one fundamental biological building-block of Aristotle’s ethics and politics. (shrink)