In 1805, Goethe published his translation of Diderot’s novelistic dialogue ‘Le Neveu de Rameau’ as ‘Rameau’s Neffe’, accompanied by substantial explanatory notes. Diderot’s highly original contribution to the period of the French ‘Lumières’ was thus first made accessible by Goethe’s German translation and it was only later, in 1821, retranslated back into French. This odd reversal of translation and retranslation stands received ideas about originality and adaptation on their heads. Moreover, it echoes an aesthetic principle which Goethe found in Diderot himself: the transfer of meaning from one linguistic and cultural context to another, within which one and the same sign also changes it’s meaning. I argue that Diderot himself had borrowed this principle from the great French composer Jean- Philippe Rameau, who explained musical modulation mainly by enharmonic equivalents. In my reading of ‘Rameau’s Neffe’, Goethe reinterprets Rameau’s musical enharmonic modulation, via Diderot, as a ‘poetic modulation’ (in Herder’s sense of the term) which foreshadows a very up-to-date conception of the cultural semiotics of translation.