The world of Atzilus versus Biyah:^[1]

The world of Atzilus is considered to be called by G-d’s name, as it is in this world that the infinite light of G-d actually resides. The infinite light of G-d unites with that world with absolute unity in both its light and vessels. Every aspect of that world is nullified and incorporated within the infinite light of G-d Himself. This concept is known as “Ihu Vechayohi Chad,” that He and his lights are one, and “Ihu Vegramohi Chad,” that he and His vessels are one. Notwithstanding this, the world of Atzilus is infinitely distanced from the essence of G-d himself and is considered like actual action in its comparison. It is only a mere ray of infinity that descends below and unites with the world of Atzilus without any separation until the entire world is filled with this ray, and nothing else can independently exist there other than this ray. The word Atzilus comes from the word Vayetzal which means to extend and separate, as in this world G-d extends his infinite light and separates a ray from it so it resides in it. The vessels of this world are there to define the various divine lights to specific jobs and focuses, although nonetheless does not contradict the infinite Divine light that resides on them, and hence the character traits of the world of Atzilus are all considered the characters of G-d, such as the wisdom of Atzilus is considered the wisdom of G-d. This is in contrast to the world of Beriyah and down which do not enjoy this level of unity with G-d’s infinite light, and indeed G-d’s divinity is hidden and concealed in these world’s, within various garments of concealment, each to their degree and level. It is for this reason that these worlds are considered the worlds of separation, as they appear separate from G-d’s unity. Nonetheless, even in Biya”h the separate existences become nullified due to the revelation of Divine light in accordance to each world. It is the vessels which create the existence of the creations of each world, while it is the Divine light which brings about their nullification. Atzilus comes from the world of Tikunn while Biya”h comes from the world of Tohu. The purpose of the Torah and Mitzvos is to elevate the levels of Biya”h to be united with Hashem like the level of Atzilus.

[1] Torah Or p. 3A