The earliest proof that William did indeed write 37 plays was Robert Greene’s
criticism of the Bard in his Groatsworth of Wit, Bought with a Million
of Repentance which attacked Shakespeare for having the nerve to compete
with him and other playwrights in 1592 . Robert Greene made this quite clear by
calling him "an upstart crow". This criticism was placed with the
Stationers’ Registrar on the 20th of September, 1592.