Le Je ne sais quoi

Je ne sais quoi, Le. An expression which became popular in polite circles in the 17th c. to refer to elements in human affairs or art which are difficult to explain in terms of classical rationalism. It is discussed in the fifth of the Entretiens d'Ariste et d'Eugène by Bouhours, who sees in it a mysterious touchstone of refined taste.

[Peter France]