In the 18th century, the Abbe, a priest, visited a home and found two little girls who did not speak (and because of their silence, he thought they were rude). Soon he found out they were not rude, only deaf. This inspired him to invent a sign language and teach these language-less children.
To develop the sign language, he observed and learned from a rudimentary system of signs already being used by the deaf people of Paris. The Abbe's system incorporated these rudimentary signs into a more formalized sign system.
The Abbe Charles Michel's success led to a "class" of at least 40 students, and in 1754 he set up and funded by himself, the first public school for the deaf in France, the "Institution Nationale des sourds-muets de Paris," which translates to the National Deaf-Dumb Institute of Paris. The formal sign system enabled deaf people in France to communicate words and concepts, and became the basis for American Sign Language and also influenced other European sign languages.
In addition, the Abbe also published a book, Instruction of Deaf and Dumb using Methodical Sign, and a sign language dictionary.
France has honored the Abbe by putting him on a stamp that was issued in 1959. Interestingly, it seems there was a bit of a battle to get a statue of him here in the United States—the National Association of the Deaf historic film collection includes a short 1913 film of an address addressing the need for such a statue here.