Luckily for Diderot, he was thus generous by temperament, and not because he expected gratitude.

Any necessitous knave with the gift of tears and the mask of sensibility could dupe and prey upon him.

In one case he had taken a great deal of trouble for one of these needy and importunate clients; had given him money and advice, and had devoted much time to serve him.
At the end of their last interview Diderot escorts his departing friend to the head of the staircase.

The grateful client then asks him whether he knows natural history.

"Well, not much," Diderot replies; "I know an aloe from a lettuce, and a pigeon from a humming-bird." "Do you know about the _Formica leo ?_ No?
Well, it is a little insect that is wonderfully industrious; it hollows out in the ground a hole shaped like a funnel, it covers the surface with a light fine sand, it attracts other insects, it takes them, it sucks them dry, and then it says to them, 'M.