As Le Breton was not strong enough to bear the material burdens of producing a work on so gigantic a scale as was now proposed, so Diderot felt himself unequal to the task of arranging and supervising every department of a book that was to include the whole circle of the sciences.

He was not skilled enough in mathematics, nor in physics, which were then for the most part mathematically conceived.

For that province, he associated with himself as an editorial colleague one of the most conspicuous and active members of the philosophical party.

Of this eminent man, whose relations with Diderot were for some years so intimate, it is proper that we should say something.
D'Alembert was the natural son of Madame de Tencin, by whom he had been barbarously exposed immediately after his birth.

"The true ancestors of a man of genius," says Condorcet finely upon this circumstance, "are the masters who have gone before him, and his true descendants are disciples that are worthy of him." He was discovered on a November night in the year 1717, by the beadle, in a nearly dying condition on the steps of the church of St.John the Round, from which he afterwards took his Christian name.