Where Diderot was exuberant, prodigal, and disordered, D'Alembert was a precisian.
Difference of temperament, however, did not prevent their friendship from being for many years cordial and intimate.

When the Encyclopaedia was planned, it was to D'Alembert, as we have said, that Diderot turned for aid in the mathematical sciences, where his own knowledge was not sufficiently full nor well grounded.

They were in strong and singular agreement in their idea of the proper place and function of the man of letters.

One of the most striking facts about their alliance, and one of the most important facts in the history of the Encyclopaedia, is that henceforth the profession of letters became at once definite and independent.

Diderot and D'Alembert both of them remained poor, but they were never hangers-on.