Rav Shach says that both Rav Acha ben Ika and R. Abahu [in tractate Sanhedrin page 3] hold that one person can judge a case. [Meaning cases between two individuals. The issue before Rav Acha and R Abahu is judging cases of civil law that come up between people. Not only that, but there can be cases where even the great Sanhedrin is wrong and has to itself bring sacrifices for making wrong judgments in cases involving karet [cutting off.] In those cases, if an individual knew better than the Sanhedrin then he is required to bring his own sacrifice, for he should not have followed the Sanhedrin when it was wrong. Thus we learn that objective morality does not depend on what the Sanhedrin says or on what anyone says. Objective right and wrong means that what is right is right, no matter what anyone says.]
At any rate, there is a debate between Rav Acha and R.Abahu if one person can be called a "court of law" [beit din] for laws that relate to a court of law, like admission before a court is different than admission outside a court.
You can see the point of Rav Shach right away in the Rambam Laws of the Sanhedrin chap. 2 [paragraph 10] where it says that a court of law is not less than three people even though one person can judge a case. Some rishonim (mediaeval authorities)[e.g., Rashba] thought that means the Rambam poskined/decided like Rav Acha but Rav Shach says that to both Rav Acha and R Abahu, one can judge but to R Abahu he is not a court of law. The proof that Rav Shach is right is that the Rambam writes, "If one judges that is Ok from the Torah, but from the words of the sages we need three. And two that judge, their judgment is not a judgment." If the Rambam would be poskening like Rav Acha ben Ika, then he would say two that judge, their judgment is a judgment. [To R. Abahu their judgment is not valid and that is what the Rambam wrote there so it does look at least in this place that the Rambam is going like R Abahu.] So he goes like R. Abahu and that shows that even R Abahu holds that one can judge from the Torah but is not considered a court of law.
However clear this might be in this place, still Rav Shach brings a few questions on this idea that I think would have given reason to the Rashba, the Ran, and the Keseph Mishna to say that the Rambam in fact poskined like Rav Acha.
The question of Rav Shach about his own idea is this: R. Abahu holds "mixing of paragraphs" [eruv parshiot] so that all civil law needs a court of law of three. One is not a court of law to him. Then why is Iraq/Bavel [Babylon] can a civil court judge only cases of admissions or loans? That we can understand to Rav Acha who makes a difference between them and cases of injury or theft which are cases the Torah says needs a court of three people with ordination from mount Sinai. [That is the authentic ordination.] But To R. Abahu all cases are like that, and are judged in Iraq [anywhere outside of Israel] only because of the courts there being representatives of the court in Israel. But to Rav Acha there is no representation. They judge because they are a court --just not one that can judge cases where the Torah requires explicitly three judges with the authentic ordination which can not exist outside of Israel.
What is an issue here for me is that "mixing of paragraphs" [eruv parshiot] comes up in Bava Metzia circa page 98. It seems if we hold of such a principle in one place we ought to hold of it elsewhere also.
