This article scrutinises one of the most challenging and provocative theses of Nietzsche’s Birth of Tragedy, that only as an aesthetic phenomenon can existence and the world be (or appear to be) ‘justified’. The thesis, which appears in two subtly different versions (together with several related formulations) and was highlighted in the ‘Attempt at Self-Criticism’ prefaced to the new edition of 1886, is enmeshed in the intricate web of ideas by which Nietzsche constructs both a general aesthetics and a specific theory of tragedy. Through a close examination of various strands of the work, the article analyses Nietzsche’s frequently masked revaluation of a series of Greek sources of thinking, not least his ‘inversion’ of both the metaphysics and the aesthetics of Plato’s Republic. Above all, the thesis of aesthetic ‘justification’ is shown to be 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 yet exhilarating insight into reality, even in its darkest features, as itself the creative work of a ‘world-artist’, the latter allusively associated by Nietzsche with the philosophy of Heraclitus.