In Bava Batra on page four there is a Rashba that is brought in the Shita Mekubetzet that asks why does the Gemara ask "why do we need the mishna? Is not the law of the mishna simple"? The Rashba's question is perhaps it could be a case of one party says "I built half the wall", and the other party claims "I built the whole wall". Would not the law then be one takes 3/4 and the other takes 1/4? So the law of the mishna would not be simple. We would need it to tell us that since they are both required to build it we say it both 1/2 to each. The answer of the Rashba is that since the place belongs to both, it is simple that we would divide 1/2 for each even if one would say, "I built it all," and the other would say, "I built 1/2."
This seems to go along with the Rambam. While the mishna says since they both have to build the wall, they divide the stones and the place equally if it falls. The Rambam says, since the place belongs to both, therefore they divide the stones. That seems like a contradiction. The mishna hangs the splitting of the place and stones on the fact that they both have to build the wall. The Rambam hangs the splitting of the stones on the fact that the place belongs to both. But it does not have to be a contradiction. It could be that the mishna is just saying the same thing in a shorthand way. And that is apparently exactly what the Rashba and Rambam mean. That the place is known to be of both and that is what causes the stones to be divided equally. [And why would the place be more simple than the bricks? Rav Shach suggests that the Rashba holds like Tosfot that the place is a case of D'rara DeMamona. That would answer why if one says he has 1/2 and the other all, they still divide by half.]
[All this is what I gathered from reading Rav Shach's explanation of the Rashba. ]
[The question that I have is that if this is the idea of the mishna, then it is a round about way of putting it.. It is saying: Since they both have to build the wall, therefore the ground belongs to them both. And then then the bricks belong to both. I mean what does building the wall have to do with the ground? But based on the Rashba I can see what would have forced the Rashba and the Rambam to understand in this way. That is the question of the Gemara, "Is it not simple?"]
