He used to say, according to Rabbi Yishmael, about judges: “Don't be a person who judges alone,” even though, when talking about monetary cases, you usually need three dayanim, three judges, to be on the beit din (rabbinical court). If you have somebody who's a mumcheh—a mumcheh is an expert—he's allowed to be a judge on a case alone.
But what the Mishnah is saying is that he shouldn't be dan yechidi, even though he's technically allowed to. He shouldn't be a judge who does it alone; he should have other judges join him and have other people's opinions involved, no matter what, even though technically he's allowed to do it alone, because the only person who could judge alone is really the Echad, the one, who is HaKadosh Baruch Hu. Only Hashem, with all of the wisdom in the world, could judge alone; there's no need for him to add judges. But for us, human beings who have limited capability, automatically having other people judge and having other opinions around will help us.
And if you have other judges join you on the beit din, and you're the mumcheh, you're the expert, you can't tell them, “Okay, you're going to join me in order to just share the responsibility, but accept my opinion. You have to do what I tell you, because I'm the expert judge.” You're not allowed to tell them that. They're permitted to tell you. If they're the majority opinion, you want them as part of the beit din, and they have the majority, they're allowed to tell you that they want you to accept their opinion, because they're the majority. But you're not allowed to force them to accept your opinion, because you're not the majority; you're just one judge.