In our Gemara on Amud Aleph, Rav Ami makes a declaration in the middle of a dispute: “Even if Yehoshua bin Nun said it to me, I would not listen to him!”

As a rhetorical statement of doubt, it would make more sense to say, “Even if Moshe said it to me…,” or to reference a future messianic time of Torah knowledge, “Even if Elijah the prophet said it to me…” What is the significance of using Yehoshua? This seems to be a deliberate choice.

Shulchan Aruch (YD 242:37) rules that it is forbidden to declare “Even if Moshe would tell me…,” and it is considered such a great disrespect that the person should be flogged. The GR”A (ibid.) cites our Gemara as the source. Presumably, Rav Ami avoided saying Moshe and chose Yehoshua instead because otherwise it would be disrespectful. How is it disrespectful to Moshe but not to Yehoshua?

We might say it is irreverent to grab Moshe’s name in a casual manner. The problem with that is Yehoshua should deserve similar respect. The Hagahos of Chasam Sofer on Shulchan Aruch (ibid.) quotes a Gemara (Eiruvin 18b) which states:

“The Sages taught: With regard to one who counts out money for a woman from his hand into her hand or from her hand into his hand, in order to lustfully gaze upon her, even if in other matters he is like Moses our teacher, who received the Torah from Mount Sinai, he will not be absolved from the punishment of Gehenna.”

Here the Gemara itself uses Moshe in its text. Therefore it is not just dropping Moshe’s name that is disrespectful.

The Taz (ibid.) says it is because it sounds like someone comparable to Moshe could come along, which is impossible because there is no prophet like Moshe (Devarim 34:10). This is difficult, but it seems that he understands the phrase to be, “Even if someone as great as Moshe told me…”

The Meshech Chochma in the beginning of Shemos interprets it differently. Since Moshe is the only prophet who can transmit new Torah laws, the statement of Rav Ami chose Yehoshua specifically because he meant to say, “Even someone as great as Yehoshua cannot tell me new Torah. And in terms of logic, the Torah is not in heaven (Bava Metzia 59b).” However, to say the same thing about Moshe would be disrespectful because indeed Moshe can transmit new halachos. This also seems to be the interpretation of the Rambam, in his introduction to his commentary on the Mishna.

The Chasam Sofer on our Gemara says a different explanation. Rav Ami was exaggerating and exclaiming that even if somebody as wise as Yehoshua quoted this halacha to him, because it sounds so wrong he would assume he misheard it from his rabbi or misremembered it. While somewhat irreverent, it is tolerable because it is still possible to say that Yehoshua, as a human being, could make a mistake, and therefore if Rav Ami had a tradition or found what was being told to him ludicrous, he would assume that the person made a mistake. However, the same thing cannot be said about Moshe—that he misheard it—because Moshe’s rebbe, so to speak, is God. And we must trust implicitly that Moshe did not misunderstand or confuse anything that he transmits as a prophet.