See also:
http://www.torah.org/learning/hamaayan/5767/vaera.html.
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Rashi comments: "He commanded them regarding Bnei Yisrael"- to deal with them in a gentle manner and to be patient with them. "Regarding Pharaoh, the king of Egypt"-that they should show respect to him in all they spoke.
R' Avraham Yitzchak Bloch z"l hy"d (Rosh Yeshiva in Telz, Lithuania; killed in the Holocaust) asks: Why did Moshe and Aharon need to receive a special command to deal with Bnei Yisrael in a gentle manner? Moshe and Aharon were, after all, exceedingly humble and undoubtedly treated every person respectfully.
R' Bloch explains: There are two concepts that come into play when one is seeking the truth. One is "emet la'amitah"/ "absolute truth." The second is simply "emet" / "truth." Unlike emet la'amitah, ordinary emet is not pure in the sense that one who wishes to impart emet may alter his presentation to account for his listeners' backgrounds and preconceived notions. But such a presentation is not without risks, for it may lend credence to the very notions that it seeks to refute. Indeed, the mere fact that one would trouble to defend Torah beliefs against heretical ideas gives credibility to those very heretical ideas.
Returning to our verse:
Why would Hashem command Moshe and Aharon to show honor to the evil Pharaoh? Absolute truth would dictate that Pharaoh did not deserve honor, but relative truth required that he be honored. Honoring kings, whether or not they personally deserve honor, is necessary to preserve world order. Therefore, such honor is the "truth." Rashi himself notes this when he explains why Yaakov sat up in Yosef's presence. He writes (Bereishit 48:2): Yaakov said, "Although he is my son, he is a king, and I will do honor to him." >From this, Rashi continues, we may infer that we must show honor to a person of royal rank. Similarly, he concludes, Moshe showed honor to the king..
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