The art of the parenthesis is one of the great secrets of eloquence in society.

[L’art de la parenthèse est un des grands secrets de l’éloquence dans la Société.]

Nicolas Chamfort (1741-1794) French writer, epigrammist (b. Nicolas-Sébastien Roch)

Products of Perfected Civilization [Produits de la Civilisation Perfectionée], Part 1 “Maxims and Thoughts [Maximes et Pensées],” ch. 3, ¶ 243 (1795) [tr. Sinicalchi]
    (Source)

(Source (French)). Alternate translations:

The art of the parenthesis is one of the great secrets of social eloquence.
[tr. Mathers (1926)]

The art of parenthesis is one of the great secrets of eloquence in society.
[tr. Merwin (1969)]