About the same time (1745) he published Principles of Moral Philosophy, or an Essay of Mr.S.on Merit and Virtue.

The initial stands for Shaftesbury, and the book translated was his Inquiry concerning Virtue and Merit.
Towards the same time, again, Diderot probably made acquaintance with Madame de Puisieux, of whom it has been said with too patent humour that she was without either the virtue or the merit on which her admirer had just been declaiming.

We are told that it was her need of money which inspired him with his first original work.

As his daughter's memoir, from which the tale comes, is swarming with blunders, this may not be more true than some of her other statements.

All that we know of Diderot's sense and sincerity entitles him to the benefit of the doubt.
The Philosophical Thoughts (1746) are a continuation of the vein of the annotations on the Essay.