The Mishnah before us - chapter 1, mishnah 2 of Maseches Horayos - deals with a case in which the court issued a mistaken ruling of permission, most of the people acted according to it, and afterward the court realized that they had erred and retracted. An individual who did not know of their retraction, and continued to rely on the original permission and thereby transgressed - what is his law?
We must first recall what we learned in the previous mishnah: although we followed there the opinion of Rabbi Yehudah, who holds that an individual who relies on a ruling of the court never brings a chatas, the halachah does not follow him. The halachah follows the Sages, that an individual does bring a chatas when he was not counted among the majority of Israel who committed the transgression. All the Tannaim who disagree in our mishnah hold like the Sages and follow the path of the halachah.
The words of the Mishnah:
"Horu beis din" - the Sanhedrin ruled that something carrying a penalty of kareis is permitted, for example that one may tie a double knot in one's shoelaces on Shabbos. On the following Shabbos, most of the community acted according to this ruling.
"V'yad'u shetau v'chazru bahen" - the court investigated the matter and understood that they had erred, and issued a new ruling reversing their original permission: one may not tie a double knot in one's shoelaces on Shabbos.
"Bein shehevi'u kaparasan uvein shelo hevi'u kaparasan" - whether they had already brought the par he'elem davar or whether they had not yet brought it.
"V'halach v'asah al pihen" - an individual who did not hear of their retraction from the leniency, and on the Shabbos after the retraction tied a double knot in his shoes relying on the original ruling.
"Rabbi Shimon poter" - that individual who erred and relied on the court is exempt from bringing a chatas.
An explanation of Rabbi Shimon's position: although as a matter of halachah an individual does bring a chatas, and even an individual who transgressed while relying on the Sanhedrin is obligated in one - that is when most of the people did not act according to the mistaken ruling - here, since it was the majority of the people who erred, the one who relies on the court is exempt from the chatas. And it makes no difference whether the court brought the par he'elem davar or not.
Rabbi Chaim Halevi's question:
Rabbi Chaim Halevi asks: it is understandable that the par he'elem davar comes in place of the individual's chatas and atones, but a korban does not atone for a future act, only for what has already been done. How, then, is it possible that the Sanhedrin brought the par he'elem davar, and afterward a person transgresses and is atoned for by that same korban that was already brought? Such a thing is impossible.
From here Rabbi Chaim derives both the plain meaning and the lomdus of the mishnah: the exemption does not stem from the power of the korban, but from the fact that the individual who relies on the Sanhedrin together with the rest of Klal Yisrael is considered included within the community, and therefore he is exempt from an individual's chatas - whether the par he'elem davar was brought or was not brought.
Rabbi Eliezer's position - a doubt:
Rabbi Eliezer is in doubt regarding the law of this person. On the one hand, he relied on a ruling of the court that most of the people followed; on the other hand, the court had already retracted, and if it was in his power to clarify the matter, he was obligated to clarify it, and he cannot exempt himself with the claim that he did not know the halachah. There is therefore room to say that his act was a form of negligence, and if he was negligent he is exempt from bringing a chatas; whereas if he truly was not negligent, since in fact he did not know, then this is a case of shogeg and he is obligated in a chatas.
Since the matter cannot be decided, we resort to the separate korban that was mentioned in the introduction to the tractate - the asham talui. This korban comes when a person stands in doubt and does not know whether he became obligated in a chatas or not: he cannot bring a chatas out of doubt, but he brings an asham talui, which holds the outcome in suspension. Therefore Rabbi Eliezer holds that in this case that individual brings an asham talui.
"Eizehu safek": who is the person standing in this doubt?
"Yashav lo b'toch beiso" - he remained in Jerusalem at the time when the Sanhedrin ruled leniently and then retracted, and it was within his power to go to the court and find out the updated ruling. Therefore "chayav" - he is obligated to bring an asham talui because of the doubt.
"Halach lo lim'dinas hayam" - he left the city and stayed overseas, and in the days before modern communication he had no way whatsoever of knowing that the Sanhedrin had changed their ruling. In this case Rabbi Eliezer agrees with Rabbi Shimon that he is exempt from any offering; and since he is certainly exempt from the chatas, there is no room for an asham talui either, for he is not in a state of doubt at all.
The words of Rabbi Akiva:
"Amar Rabbi Akiva: modeh ani b'zeh" - Rabbi Akiva agrees with the basic principle, that there is a situation of doubt which obligates an asham talui, but he differs regarding the precise definition: when is a person considered not to have been negligent, because there was no reason to expect him to find out that the Sanhedrin had retracted and that the act was forbidden.
He agrees about the two extremes: one who left the Land - it certainly could not have been expected of him to know about the change in the ruling, and therefore he is a shogeg, and he is included with the rest of the community who committed the transgression, and he is exempt from a chatas and from any offering, and there is no doubt here. And one who sat in the city with folded arms - it was incumbent upon him to find out the updated ruling that had once again forbidden the act, and therefore it is a doubt whether he is a shogeg or close to a meizid, and he is obligated in an asham talui.
But Rabbi Akiva adds an intermediate case: a person who is still in the city, but is preoccupied with preparations for setting out on a journey outside the Land. He cannot be expected to keep checking the rulings of the court at every moment, since he is busy with his preparations. For this reason he is not considered close to a meizid but close to a shogeg, and since the majority of the community erred just as he did beforehand, he is included with them and exempt from any offering, including a chatas and an asham talui. This is the meaning of "modeh ani b'zeh shehu karov l'hipater" - he is closer to exemption than to obligation.
Ben Azzai's question:
"Amar lo Ben Azzai: mah shanah zeh min hayoshev b'veiso?" - after all, both of them are sitting in Jerusalem, and both of them could get up at any moment, go to the Sanhedrin, receive the updated ruling and discover that the act is forbidden.
Rabbi Akiva answered him: "Shehayoshev b'veiso efshar hayah lo sheyishma" - the one sitting at home in Jerusalem has time available to him all along, and what could be better or more fitting for him than to update himself on the rulings of the Sanhedrin. But the one who is preoccupied and about to set out on a journey - "lo hayah efshar lo sheyishma", for it was not within his power to go and hear the updated ruling, because of the pressures of preparing for his trip.
In summary: in this mishnah we learned the law of an individual who acted on the instruction of the court after they had retracted: Rabbi Shimon exempts him entirely, because he is included in the community that acted according to the ruling - and as Rabbi Chaim HaLevi explained, the exemption does not depend on an offering having already been brought. Rabbi Eliezer places him in doubt and obligates him in an asham talui when he sat at home and could have found out, and exempts him when he went overseas. And Rabbi Akiva added the intermediate case of one preoccupied with setting out on a journey, who is likewise exempt from any offering, since "lo hayah efshar lo sheyishma".