Generally speaking, tannaim can argue with tannaim, and amoraim can argue with amoraim. Amoraim – who are perceived as being further from the source of the halakha than tannaim – cannot argue with their predecessors. Thus, one of the methods used by the Gemara to establish halakha is to examine the opinions of the amoraim in the face of statements of tannaim.
On today’s daf, the Gemara attempts to determine whether the halakha follows the position of Rav or Shmuel (in their disagreement about how to interpret the Mishna’s ruling about the individual who declares that he is establishing his Shabbat “beneath the tree” that is 2,000 amot away – see daf 49) by quoting baraitot that appear to support one or the other.
A baraita was taught in accordance with the opinion of Shmuel. If one erred and established an eiruv in two directions at once, for example, if in his ignorance he imagined that it is permitted to establish an eiruv in two directions, that he may extend the distance that he may walk on Shabbat in two opposite directions, or if he said to his servants: Go out and establish an eiruv for me, without specifying the direction, and one established an eiruv for him to the north, and one established an eiruv for him to the south, he may walk to the north as far as he is permitted go based on his eiruv to the south, and he may walk to the south as far as he is permitted go based on his eiruv to the north. In other words, the assumption is that he established residence in both directions based on the eiruv in each direction, and he must therefore take both into consideration before moving.
This seems to support Shmuel’s contention that even if the place established for Shabbat is not fully clear, the area that does fall into the person’s declaration is accessible to him.
The Gemara has no response to defend Rav against this baraita. Rather than establishing the halakha like Shmuel, though, the Gemara responds Rav tanna hu, u’palig – Rav has the status of a tanna, and can argue. Some understand this statement to mean that Rav was considered so important that he was permitted to disagree with the opinion presented in the baraita. It is likely, however, that the Gemara is saying that Rav really was a tanna! Rav Hai Gaon claims that Rav’s opinion actually appears several times in baraitot, under the name “Rabbi Abba” (see daf 12). Nevertheless, Rav is considered, at the same time, an amora, in that we find that his contemporaries who were first generation amoraim (e.g. Shmuel and Rabbi Yohanan) argue with him, and that their positions are sometimes accepted as the halakha. Still, his position cannot be disproved by a baraita. It is generally accepted that the Gemara only uses the answer Rav Tanna hu, u’palig when it does not have a substantive response to the question.