After being rejected by B'nei Yisrael "out of anguish of spirit and hard work," Moshe Rabbeinu is instructed to appear once again before Paroah. Moshe responds with a logical argument, (6:12) "behold, B'nei Yisrael have not listened to me. How then will Paroah listen to me for I am of uncircumcised lips?" Rashi comments that this is one of the 10 instances of the use of á fortiori argument, better known to most as kal vachomer, in the Torah. The full listing is discussed in the midrash (Bereishis Rabba 92). However, many ask that this kal vachomer does not follow logically. The Torah tells us exactly why B'nei Yisrael did not listen Moshe. If this reason did not apply to Paroah, then Moshe's logic appears faulty.
Sefas Emes takes a very practical approach to this difficulty, one with which Rabbeinu Tam preceded him by many hundreds of years. The Torah may tell us why B'nei Yisrael did not listen to Moshe, but Moshe, at the time, was not necessarily aware of that reason. Without the knowledge of B'nei Yisrael's inner feelings, Moshe's kal vachomer did, in fact, follow logically.
R' Yaakov Weinberg, zt"l, offers a deeper insight into Moshe’s logic. When Moshe Rabbeinu came before B'nei Yisrael to lead them out of bondage, they should have come to a realization of their importance and sanctity for which they merit such a great deliverance. They chose, instead, to spurn this opportunity and reject Moshe. If B'nei Yisrael could not come to realize their own sanctity and merit, argued Moshe, how could Paroah possibly come to this discovery?
(It occurred to me recently that it is rather strange that we are able to stop at the end of this pasuk (8) when Va’eira is read during the week and at Mincha. This is indeed one of the more depressing moments in the saga of our subjugation in Mitzrayim, especially for those who don’t know the ending. Moshe is supposed to be leading a great deliverance and even his own people won’t listen to him. How is it permitted to end an aliyah on that note?)