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In the distinction Rousseau brings up between physical objects and moral objects of taste in his Emile, book IV, he generalises the reasons put forward since 1749 throughout the many texts dedicated to music, in favour of melody rather than harmony. If in his anthropology, the strength of musicis proven to be conveyed by singing, it is however very important to question how a philosophy of musical expression can define its object. Rousseau has to defend the idea of how harmony, as a « physical thing », contributes to the effect produced by music, but he is inevitably conducted to review the gnoseological principles of his strict dichotomy, in order to study the real conditions of musical perception.