Eiruvin 35 - 41
A Warning to Teachers
A Jew is forbidden on Shabbos and Holidays to walk more than 2000 amos from his place of dwelling at the onset of the day. If he has a need to go somewhere beyond that techum he must make an eruv techumin - or eruv for short - before the onset of the day. By placing food at a location before Shabbos somewhere between his residence and the techum, he theoretically establishes his residence there and may walk 2000 amos from there on Shabbos.
He may sometimes find himself in a situation where he is not certain in which direction he wishes to have this freedom of movement. A report has come that a Torah scholar will be giving a lecture somewhere outside the city, but it is not certain whether it will be to the east or west. He may therefore place an eruv to the east and another to the west and declare, "If the scholar arrives in the east, I wish the eastern eruv to establish my residence; but if the scholar arrives in the west, then I wish the western eruv to establish my residence."
What if two different scholars arrive, one in the east and one in the west, and one of them is his teacher? The majority view of the Sages is that he can choose either direction, because implicit in his declaration is that if scholars arrive in both places, he can choose which eruv should be in effect. Rabbi Yehuda, however, contends that since one of them is his teacher, he definitely intended to go in this direction. The Sages counter this argument by saying that sometimes a person prefers to hear the discourse of his colleague to that of his teacher.
This seems to echo what Rabbi Chanina declared (Mesechta Taanis 7a): "Much have I learned from my teachers; even more from my colleagues and most from my students."
A note of caution, however, is sounded by Iyun Yaakov in regard to this progression. One must follow this order in exact fashion, first absorbing knowledge from his teachers and sharpening it in discussion with his colleagues before presuming to pass it on to students. He sharply criticized people in his generation (over 300 years ago!) who assumed the role of teacher before completing their own education, and who ended up confusing both themselves and their students.
The Obscure Holy Day
Rosh Hashana is also Rosh Chodesh of the month Tishrei. Yet we make no specific mention of Rosh Chodesh in our shemone esrei prayers as we do on all the other days of the New Moon throughout the year.
The reason for this is that the Torah refers to both Rosh Hashana (Vayikra 23:24) and Rosh Chodesh (Bamidbar 10:10) as days of Divine Remembrance, and therefore one mention of remembrance is sufficient for both of these special dates.
There is a difference of opinion regarding mentioning in our mussaf prayer the Rosh Hashana and Rosh Chodesh sacrifices offered in the Beis Hamikdash. One approach cited by the Beis Yosef (Shulchan Aruch Orach Chaim 591:2) as the custom of Sephardic Jews is that no mention is made of any of those sacrifices (or those of any other holiday) beyond stating that sacrifices were brought as the Torah commanded. The reason is that explicit mention of the sacrifices is made only on Shabbos and Rosh Chodesh which are frequent enough for us to be familiar with the text of the sacrifices, and not run the risk of becoming confused. (Since Rosh Hashana sacrifices are not mentioned for this reason, the Rosh Chodesh ones are also deleted to avoid creating the impression that the Rosh Chodesh dimension of the day is more important.)
Ashkenazic Jews, however, follow the ruling of Rama and cite the specific sacrifices on every holiday, and do so also in regard to the Rosh Hashana mussaf sacrifices. In regard to the Rosh Chodesh sacrifices, however, the approach of Tosefos prevails, which is that no mention is made of their specific nature beyond the mention that there were sacrifices offered in honor of Rosh Chodesh.
Another reason for obscuring Rosh Chodesh is to avoid the danger of people assuming that the second day of Rosh Hashana is the real holiday just as the second day of Rosh Chodesh throughout the year is the real first of the month. Another is to highlight the Rosh Hashana component of the day. Finally there is a reference in Tehillim (81:4) to Rosh Hashana as an "obscured holiday" which hints that the Rosh Chodesh component be obscured in our prayers.