Final answer:
William Stokoe proved in the 1960s that American Sign Language is a structured linguistic system with its own syntax and grammar, legitimizing it as a true language.
Step-by-step explanation:
In the 1960s, William Stokoe, a professor at Gallaudet University, proved that American Sign Language (ASL) is an organized, structured linguistic system. Stokoe's groundbreaking work demonstrated that ASL had its own syntax and grammar, and thus, it was a legitimate language on par with spoken languages.
His work was instrumental in changing the perception of ASL and was critical in advocating for the Deaf community's linguistic rights.