מפני שיבה תקום "Before [the elderly] shall you rise (Wayyiqra 19:32). The use of the word "Mippenei" (מפני) to mean before or in front of, seems unusual. "Lifnei" means in front of, whereas "Mippenei" usually means on account of.
The Dibrei Mordekhai quotes the Ben Ish Hai as saying that in the matter of the rebellion of Qorah, it says ויקמו לפני משה "And they rose in front of Moses" (BaMidbar 16:2). Even though they were rebelling against him, it was not possible that they would not stand "on account of" (Mippenei) his honor. So what did they do? When they spied him approaching from a distance, they immediately rose, so that when he approached they would already be standing.
In view of this, the Dibrei Mordekhai says that this explains the Torah commandment of rising "on account of" an elderly person. It comes to tell us that you must rise on account of him, when he is close to you, so that it will be apparent to all that your rising was because of him.
(See Dibrei Mordekhai, Wayyiqra, Parparaoth)
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