Now the Alter Rebbe explains the reason why this “retraction” of the expression of the Infinite Light was needed:
The worlds were unable to receive the revelation of Hashem’s Infinite Light as it existed previously within Himself, and they would have lost their own seemingly independent existence.
Therefore, Hashem retracted the Light so that it should be included within “the Luminary,” (so that it exists only in potential, but not actually revealed,) and thereby concealed it, so that the Light would not be revealed, rather, only “the Luminary” would be “revealed.”
(However, in “the Luminary the Infinite One Himself, there is no possibility of Tzimtzum-Withdrawal or concealment, G-d forbid. On the contrary, “the Luminary” is revealed even more than before the Tzimtzum.)
