In the disagreement between Rabbi Yohanan ben Nuri and the hakhamim (Mishna, 45a) the hakhamim limited the movement of the person who awakened on Shabbat to the surrounding four cubits. The Gemara now returns to the opinion of the hakhamim and asks for the source of four cubits as being the limit for someone who is not allowed to move at all.
The Gemara inquires about the basis of this law: These four cubits within which a person is always permitted to walk on Shabbat, where are they written in the Torah?
The Gemara answers: As it was taught in a baraita: The verse “Remain every man in his place; let no man go out of his place on the seventh day” (Shemot 16:29), means one must restrict his movement to an area equal to his place.
The passage that is presented as the source for this rule is shevu ish tahtav – “remain every man in his place,” which refers to the time when the manna fell and the Children of Israel were instructed to refrain from going out to collect it on Shabbat. The word tahtav literally means “under him,” and the Gemara presents this as the source for limiting movement to the area of a person lying down – three cubits – together with another cubit that allows him room to stretch out his arms and legs (according to Rabbi Meir) or to move an object from beneath his feet and place it under his head (according to Rabbi Yehuda).
In an attempt to clarify this rule, Rav Mesharshiya tells his son to ask Rav Pappa whether the amot under discussion in the Mishna are subjective and differ for each person (the word ama means an arms-length – the distance from his elbow to the tip of his index finger), or if they are an objective size that is the same for everyone. Rav Mesharshiya supplied him with follow-up questions to ask on whatever answer he received. Were he told that amot are objective, he was to ask whether the same size would apply to Og the king of Bashan (see Devarim 3:11); were he told that amot are subjective, he was to ask why this rule does not appear with other such rules in the list (Mishna Kelim 17:11) of halakhot that differ from one person to another.
Rav Pappa is, apparently, taken aback by the question, and responds that were we to try to read Mishnayot so closely in an attempt to infer such things, we would never have time to learn. This reaction probably stems from the fact that Rav Mesharshiya’s questions do not stem from the Mishna itself, but are based on an attempt to read things into the Mishna that are not clearly indicated there. Rav Pappa is suggesting that such attempts to read into the Mishna are misguided, since there may be a variety of reasons for a particular phrase to be chosen for use in the Mishna – based on style, for example – and trying to extract a halakha from such an inference may lead to mistaken conclusions or internal contradictions.
Rav Pappa does answer the question, though, and rules that an ama is subjective, based on the size of the person. However, there are exceptions, such as a person whose arms are disproportionately small for his body, where the amot will be based on the standard, objective measurement of an ama. This is why the rule does not appear in the list of subjective halakhot in the Mishna Kelim.