This thesis is in two sections. The first critically examines the tradition of harmonia mundi. The earliest complete and developed account of musica mundana appears in Plato, but numerous fragmented references appear in the pre-Socratic sources. The notion that harmonia mundi originated as an idea of quantitative speeds and distances of the celestial bodies, is discredited. Rather, it is shown that it more probably originates as an expression of an 'esoteric spiritual' teaching in which self-knowledge, death, the concept of harmonia, and consequently music, are related. The idea that the greatest importance of music rests on the relationship of music and emotion, is undermined in this context, and the relationship of music, emotion and experience is examined in a way that supports much of what was asserted by Hanslick in the nineteenth century. The interpretation of ancient sources is critically assessed in terms of common 'hermeneutic filters' which are shown to be inconsistent with the content and hence context of some of the sources. It is also argued that Plato should be approached not merely through the assessment of the arguments that appear in his discourses, but in the light of his portrayal of the life and death of Socrates. The discourses are treated as inexact, exoteric expressions of esoteric meaning, much of which can be gleaned from the 'symbol' and example of Socrates' own life and death. The second section presents an original music philosophy that is an entirely contemporary exposition of the essential meaning of the harmony of the spheres tradition, as interpreted in the first section. In this contemporary exposition, some of the ideas that appear in Plato concerning the relationship of soul, world and harmonia, are re-expressed in terms of self, world, and an original contemporary 'parable' for harmonia. The background to the ancient tradition, of the macrocosm-microcosm relationship, is brought into contemporary terms, drawing critically on some ideas of the quantum physicist David Bohm, and questions raised by quantum theory in general. Finally, the nature of the macrocosm-microcosm is related to parts of Wittgenstein 's Tractatus logico-philosophicus.