This commentary on Plato’s Philebus reconciles a close analysis of the text with a new interpretation of the dialogue. In Philebum focuses on the overarching metaphysical and cosmological coherency of the dialogue rather than its ethical import. This interpretation contrasts with the more common segmented philological analysis of this most evocative of Platonic dialogues. Plato’s late ontology and theory of an immanent Good portray a very different philosophical terrain than that of the transcendental visions of the Good found in other dialogues. The final chapter of In Philebum, entitled “The Life of the Speculative Philosopher,” extends this analysis of the dialogue to contemporary speculative philosophy. Based on Plato’s portrayal of a fourfold onto-cosmology and of the Good as measurability, proportionality, and intelligence, In Philebum makes connections between the doctrine of measured order-relations, triadicity, quadratic structure, and self-explanation that figure predominantly in contemporary speculative philosophy. It is intended for a broader audience of readers of Plato as well as for graduate students and commentators on Plato. In Philebum contains a Prolegomenon on the controversies surrounding the structural divisions of the Philebus as well as an up-to-date bibliography and general index.