Blaise Pascal publishes the first of his Lettres provinciales.

Blaise Pascal was a French mathematician, physicist, inventor, philosopher, and Catholic writer.

The Lettres provinciales are a series of eighteen letters written by the French philosopher and theologian Blaise Pascal under the pseudonym Louis de Montalte. Written in the midst of the formulary controversy between the Jansenists and the Jesuits, they are a defense of the Jansenist Antoine Arnauld from Port-Royal-des-Champs, a friend of Pascal who in 1656 was condemned by the Faculté de Théologie at the Sorbonne in Paris for views that were claimed to be heretical. The first letter is dated January 23, 1656 and the eighteenth March 24, 1657. A fragmentary nineteenth letter is frequently included with the other eighteen.

January 23 is the 23rd day of the year in the Gregorian calendar; 342 days remain until the end of the year.