Every Shabbat the lehem ha-panim – the Shewbread – would be switched, with the old bread taken and eaten and the new bread placed on the table. The Mishnah on today’s daf (=page) describes how this was done, with one group of kohanim placing the new bread on the table as another group was removing the week-old – but still fresh – loaves. Replacement of the loaves in this manner was essential because the Torah commands that the loaves be placed before God tamid – “always” – (see Shemot 25:30). Rabbi Yossi disagrees with this understand of tamid, arguing that even if the loaves were removed in the morning and replaced in the evening, it would still fulfill the requirement of tamid, which should be understood as teaching that a night should not pass without Shewbread on the table.
Rabbi Ammi infers from Rabbi Yossi’s teaching that the concept of tamid – “always” – need not be understood as requiring 24 hour vigilance, and applies it to other situations, as well. Specifically, he says that Torah study, which is a requirement “day and night” (see Yehoshua 1:8) can be fulfilled by studying a chapter in the morning and a chapter at night. This suggestion is supported by the ruling of Rabbi Yohanan in the name of Rabbi Shimon bar Yohai who taught that a person fulfills his obligation to learn Torah according to the passage in Sefer Yehoshua even if he merely recites the Shma in the morning and in the evening, but that it is forbidden to teach this to unlearned people who will take advantage of it. Ravaargued that it is a mitzvah to teach this law to people who are unlearned, so that they should understand how great the reward for serious Torah study might be.