This article scrutinises one of the most challenging theses of Nietzsche's Birth of Tragedy, that only as an aesthetic phenomenon can existence and the world be (or appear to be) ‘justified’. Through a close examination of the work's frequently masked revaluation of a series of Greek sources of thinking, not least its ‘inversion’ of both the metaphysics and the aesthetics of Plato's Republic, the article shows how the thesis of aesthetic ‘justification’ is caught up in a tension between Apolline and Dionysian interpretations, the first entailing a quasi-Homeric sense that the Olympians justify human existence by living a transfigured form of it themselves, the second involving a tragic insight into reality as itself the creative work of a ‘world-artist’, the latter allusively associated by Nietzsche with the philosophy of Heraclitus.