Classical philosophers knew of and were fascinated by the ability of musicians to replicate a tune at first hearing. Dionysius of Halicarnassus noted that if you play a new tune for an accomplished musician, he will be able to play it back at you with barely any hesitation or preparation ( De Comp. 25.241-246). Philosophers' reflections on this phenomenon of the 'knowing ear' had important consequences for ancient understandings of both music and perception. In a time-span corresponding roughly to the fourth century BCE, two critical and related philosophical developments took place: a new understanding of perception emerged, and an explicit theory of music was elaborated. The result of these intertwined events was a conception of the musical ear as a sensual embodiment of rationality: it could analyse and understand musical expression without requiring any supplementary intellectual labour. Sean Gurd tells the story of how this conception came to be, tracing its developments through the works of Plato and Aristoxenus, and offers a critical assessment of the consequences for music theory today.