Sign languages are natural languages
Contrary to what many people still think, Sign Languages are human languages, just like Spoken Languages. That is, they are natural languages that have appeared spontaneously over time within a community of speakers: they have not been invented or designed artificially. They have all the abstract structural properties that we have recognized in the study of Spoken Languages. They have their own lexicons, and complex and sophisticated grammars that allow for an infinite production of sentences, just like any Spoken Language. The problem is that linguistics had not proved all these facts until quite recently, in the second half of the twentieth century, and they are still little known.