The Mishna that we have today is a collection of statements of Torah she’ba’al peh (oral tradition) collated and edited by Rabbi Yehuda HaNasi. Rabbi Yehuda HaNasi often chose the opinion that he believed to be the one accepted as the halakha and included it in the Mishna without attributing it to a particular Rabbi. Such a Mishna is called stam – a simple Mishna – i.e. one about which we find no argument.
Obviously, Rabbi Yehuda HaNasi could not include all of the statements of oral tradition in his Mishna, and many of them were later collected by his students, Rabbi Hiyya and Rabbi Oshiya. These collections were called baraitot – “outside” collections – i.e. those that were not included in the collection of the Mishna. The baraitot are often brought into discussions in the Gemara in an attempt to clarify or broaden our understanding of the laws taught in the Mishna.
Our Gemara describes a conversation between Rabbi Nahum and Rabbi Abbahu. Rabbi Nahum was serving Rabbi Abbahu and took the opportunity to ask him a series of questions about how to determine the halakha when learning Mishna.
The conversation went as follows:
Q: What if we find a disagreement followed by a stam Mishna?
A: We follow the stam Mishna.
Q: What if we find a stam Mishna followed by a disagreement?
A: We do not automatically follow the stam Mishna.
Q: What if we find a stam Mishna and a disagreement appears in a baraita?
A: We follow the stam Mishna.
Q. What if we find a stam baraita and an argument in the Mishna?
A. If Rabbi Yehuda HaNasi did not teach a clear conclusion, from where would Rabbi Hiyya, editor of the baraita know it?
The Maharik explains the argument put forward by Rabbi Abbahu as follows. We can assume that Rabbi Yehuda HaNasi was more knowledgeable than his student, Rabbi Hiyya, so if Rabbi Yehuda HaNasi was not aware of a compelling reason to rule in a particular case, we cannot assume that Rabbi Hiyya had additional information that would enable us to accept his ruling in the face of the disagreement that appears in the Mishna.