This paper was presented on Dec 13th 2017 at the Durham Classics Research Seminar Series on 'the Legacy of Plato's Timaeus'. Abstract: In many passages of Plato’s dialogues, the soul is depicted as a ‘harmonious’ whole – a notion that was already attested in Pythagorean thought but was to be significantly criticised and modified in Plato’s own works, starting at least from the Phaedo. But it is in Book 4 of the Republic that the most important ethical values of the ideal city, and the souls that embody them, are depicted in thought-provoking musical terms: in fact, moderation (sōphrosýnē) and justice (dikaiosýnē) are repeatedly described as a symphonic agreement of sounds (συμφωνία, Resp. 4.430e, 4.431e–432b) or even an actual ‘tuning system’ (ἁρμονία, Resp. 4.443c–444a). This paper will focus on Plato’s fascinating depiction of justice as special kind of harmonía that epitomises the best possible organisation of the soul, exploring his nuanced use of the model of lyre tunings in performative, theoretical as well as educational terms. More specifically, by comparing Plato’s use of harmonic imagery with technical discussions of lyre tunings and their key role in educational settings, I will show how Plato exploited distinctive features of traditional Greek lyre harmoníai to give shape to his innovative understanding of the structure of the soul and the harmonious, but not strifeless, relationship between its individual components. In the second part of this paper, I will look at how the model outlined in the Republic sheds light on the musical structure that gives shape to the World Soul in the Timaeus, advancing a new interpretation of its elusive harmonic organisation. More generally, this chapter aims at showing how Plato’s harmonic characterisation of the structure of the soul is essentially based on his (and his readers’) acquaintance with basic practical as well as theoretical features of these musical models. Hence, musical theory and practice represented a shared repertoire of concepts that Plato could embrace to express his innovative philosophical notions in terms that his readers could easily relate to on the basis of their own aesthetic, emotional, as well as cognitive experience.