At the end of Episode I, we see Yoda telling someone (I think Mace Windu) the Sith Rule of Two; "Two there will be. A master and an apprentice. No more, no less." However, that rule was finalized by Darth Bane, who was believed dead at Ruusan. How did Yoda know the rule, if the Jedi thought that it could not exist?

Your thoughts would be appreciated,
MTFBWY