[If] this one says: “This is my handwriting, and this is the other’s handwriting,” and this one says: “This is my handwriting, and this is the other’s handwriting,” these are believed. [If] this one says: “This is my handwriting,” and this one says: “This is my handwriting,” they must join to themselves another person; [these are] the words of Rabbi. The Sages, however, say: They need not join another person to themselves; rather, a person is believed to say: “This is my handwriting.”