Chapter 1, Mishna 1
If for Shabbos carrying we’re adjusting an alley that has a crossbeam less than 10 tefachim, handbreadths, above the ground by lowering the ground itself, how far should our digging extend into the alley? Rav Yosef says 4 tefachim, Abaye says 4 amos, cubits. To understand their dispute, we look at R’ Ami’s analysis of an alley whose side wall was breached. Permission to carry depends on how wide is the breach, and how close to the end of the alley. Abaye seems to reject R’ Ami’s ruling that that 4 tefachim of wall still standing near the end of the alley is enough, but Abaye could answer that we can’t learn out a correction to an alley that was never valid for Shabbos-carrying from one that was valid and then became damaged. Instead Abaye looks to the definition of an alley itself. It must be longer than it is wide and capable of receiving the openings of at least two courtyards that open into it, with each courtyard containing two or more homes. You need at least 4 amos of depth to accomplish that. Next case is a sidepost that is very wide – too wide to work as a halachic marker for rendering the alley valid for Shabbos carrying. We need an additional, smaller side post as a marker unless the over-wide side post actually fills more than half the alley’s opening, in which case this is no longer an alley but a courtyard with four sides. What if half and half? Look to the great Rav Ashi, editor of the Talmud, to explain that in cases of doubt we rule leniently re a rabbinc prohibition.