I argue here for a poetics in prose at work in Heraclitus, one that makes the text an occasion for an exemplary experience of the kosmos as a form of active and intelligible organization. This project works in two directions. First, I contextualize Heraclitus’ unique form of expression by showing how it breaks from existing practices – common to both poetry and prose – of personal authority and narrativity. Second, I engage closely with Heraclitus’ writing to show how it makes the intelligible activity of the kosmos available in the local encounter with the text, partly through an analogy between the kosmos and the meaningful operations of human language. Ultimately, this poetics in the text is part and parcel of seeing philosophical understanding itself as a form of activity. Little has been made of Heraclitus’ several references to the role of the witness in archaic Greek law, but I show how these illuminate his ethical ideal of perceptive participation in the intelligible activity of the kosmos. The texts of Heraclitus thus turn out to be an important corpus for both the development of Greek philosophy and the history of Greek literature.