Jean de Muris
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Jean de Muris (1290-1351) was a French philosopher and mathematician, best known for his promotion of ars nova over ars antiqua.
Jury is out whether or not he actually wrote the Speculum musicae. That voluminous work covers the entire range of musical knowledge of the time. It defends Ars Antique and attacks the Ars Nova. Some say his authorship is unlikely, claiming that he was friends with Philippe de Vitry. That treatise of 1330 is sometimes attributed to Jacob of Liege who's views on polyphonic developments are more "conservative."
It is pretty certain (according to The New Oxford History of Music) that de Muris authored Ars novae musicae in 1319.