[If a man said:] “I know that there are nedarim, but I did not know that they could be revoked,” he can revoke [the neder]. [But if he said:] “I know that nedarim could be revoked, but I did not know that this was a neder” —- R' Meir says: He cannot revoke [it]; the Sages, however, say: He can revoke [it].