Blessings

"Vavorichah Mivarachekah Umikalalechah Ahor"; "And I will bless the ones that bless you and the ones who curse you I will curse" The Meforshim ask, just like the pasuk says, "I will bless the ones that bless you" - stating "I will bless" first, so too should the pasuk end, "I will curse the ones who curse you". Why does the pasuk reverse it by saying, "The ones who curse you I will curse?"

The Kli Yakar answers, "Machshava Tova HaKadosh Baruch Hu Mitzaref Limaaseh, Machshava Raah Ayn HaKadosh Baruch Hu Mitzaref Limaaseh"- Hashem considers a good thought (when one thinks about doing a Mitzvah) as if the person actually did it; but concerning an evil thought, Hashem only punishes when the thought come to fruition and the person commits the sin. That is the pshat in this pasuk. "Vavorichah Mivarachekah" - "And I will bless them even before they bless you. When they just have a thought to bless you I will consider their Machshava as if it was done. But Umikalalechah Ahor" - only after they actually curse you will I curse them, since Hashem only punishes when the thought come to fruition.

Vilna Gaon offers another explanation. He says that the blessing of a rich man is greater than that of a poor man, since the rich man knows what it means to have pleasure. He therefore will give a greater Bracha. On the other hand, a poor man's curse is greater than a rich man since he experienced pain in his life and can give a harsher curse. The Hashem is saying to Avraham, "Vavorichah Mivarachekah" - I will bless them before they bless you, therefore making it a blessing of a rich person who gives a more lavish Bracha. "Umikalalechah Ahor" - I will only curse them after they curse you. This way it won't be a curse of a poor man whose curse is harsh.
http://www.thejewisheye.com/rev_plechlicha.html

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