Heraclitus’s doctrine of a cosmogonic unity of opposites held together in harmoniēis the topic of “Heraclitus and the Quantum.” Like Anaximander, Heraclitus posits a self-organizing universe in which objects and agents interact to form relational wholes. It is argued that Heraclitus’s ideas anticipate physicist Niels Bohr’s atomic theory of complementarity and the systems thinking of early cyberneticists. Extended from a description of the cosmos to a prescription for living, Heraclitean harmoniē, it is argued, is tantamount to sustainability, and provides a profounder, more durable alternative to some modern prescriptions circulating under the same conceptual umbrella.