In February 1635, Mersenne said he was a master of the music of the cathedral of Saint Quentin, and in 1636 mentions him twice as a canon of the collegiate church of the same city.. Two notarial acts concerning him are known at Saint-Quentin: on 19 July 1637 he was witness to the marriage contract of Milan de Chauvenet with Catherine Le Sergent; he was also witness to the marriage of Georges de Chauvenet with Marie Le Sergent on 8 September 1641. He died in Saint-Quentin on 11 August 1658 and was buried in the Saint-Nicolas chapel of the collegiate church of Saint-Quentin.
Works
Cousu is better known as a theorist than a composer, having left an important treatise on music and a single fantasy with four parts.
The ''Système royal''
Around 1633, Cousu published a "Système Royal", dedicated to Louis XIII, probably a simple method of solfeggio which seems to have earned him some fame, and perhaps his canonicality at Saint-Quentin. That's what Annibal Gantezsuggests in 1643: ou pour Monsieur Du Cousu, qui a plus attrapé du roy avec une gamme ou un main qu’il luy a présentée, que je n’ay su faire avec mes deux pieds en parcourant tout le royaume, puisqu’il est chanoine de Saint-Quentin.... This work is lost.
This four-part piece is published in separate parts in the Harmonie universelle by Marin Mersenne, of the Cinquiesme livre de la composition de musique; he emphasizes the great know-how of the composer and the perfect correction of the writing. The fantasy is republished by Athanasius Kircher in 1650 in his Musurgia Universalis, this time in a score. Kircher also underlines the originality and value of this piece.
The ''Musique universelle''
The two mentions that Mersenne made of Cousu in 1636 reveal that he was already working on an important musical treatise at that time. This treatise was published more than twenty years later, in 1658, by Robert III Ballard, but the author's death interrupted his publication. The Musique universelle, contenant toute la pratique, et toute la théorie. Paris : Robert III Ballard, around 1657-1658. 2°, 208 p. RISM B-VI, Guillo 2003 n° 1658-F. Brussel BR : Fétis 5357 C RP. Prov. Sébastien de Brossard, with a historical note by François-Joseph Fétisat the beginning. Paris Mazarine: Rés D 4727. Prov. Abbaye de Saint-Germain-des-Prés. The only two known copies contain only pages 1-208, i. e., complete Books I and II, and the first chapters of Book III.. The book by Cousu contains a systematic presentation of the principles and rules of music, its notation and counterpoint. The work is considered honest and thorough, but it does not contain any fundamentally new ideas. But he was one of the first theorists to put theory and practice at the same level: "Practice being joined to theory, or theory to practice, is much better than when they are separated." However, Cousu was still strangely attached to explaining the ancient signs of measurements and ligatures, whereas their use was already falling into disuse. This is also apparent from a letter from Mersenne to Giovanni Battista Doni, which quotes Cousu and his work in 1635:
