Beraisa: It is permitted (to urinate) on a stone wall.

This is clear from the following story...

R. Yochanan left the Beis Knesses and urinated behind the gate, next to the wall. The students were unsure if it was because the wall was made of stone or because he was not able to hold himself back.

(The Mishnah taught that one must distance millstones from his neighbor's wall - three tefachim from the lower stone, which is four tefachim from the upper stone). This is referring to millstones in Bavel, which are ground by man; however, in Eretz Yisrael they are usually rotated using donkeys, so more space (4 tefachim) is needed around the millstone for the donkey to walk.

The Mishnah taught that an oven must be distanced from his neighbor's wall - three tefachim from the oven's base, which is four tefachim from the edge of the oven.

R. Yehudah ben Pazi: This is measured from the outer edge of the oven (meaning that the thickness of its wall is not included in the measurement).

Rabbanan: From the inner side of the wall of the oven (meaning that the thickness of its wall is included in the measurement). (Note: Many Rishonim write that the rest of the text in this section is out of place in the Gemara and should in fact be printed at the beginning of the Gemara that follows the next Mishnah, which discusses setting up an oven. See later.)