Tanya for 29 Tishrei

Now, though (34) "G-d is one and His Name is one," i.e., [though] His speech and the "breath of His mouth," which in the sacred Zohar (35) is referred to as His Name, is singularly and uniquely one, nevertheless, the radiation and efflux of life-force which issues forth from His mouth, divides into four different levels. These are the Four Worlds - Atzilut, Beriah, Yetzirah and Asiyah. (36)

The difference is due to ((25) many) contractions and screens that veil and obscure the light and the life-force, so that it will not radiate in the World of Beriah as much as in the World of Atzilut; and in the World of Yetzirah [the opaqueness is thick-ened] by means of further contractions and screens; and so on, [i.e., how much more so in the World of Asiyah].

Nevertheless, there is no change whatever, heaven forfend, in the essence of the Shechinah, which is the "word of G-d" and the "breath of His mouth."

Also, as regards the radiation and efflux of life-force [that issues from the "word of G-d]," the radiation which is in [the World of] Atzilut pierces the screen and vests itself in [the World of] Beriah. Likewise from [the World of] Beriah to [the World of] Yetzirah, and from [the World of] Yetzirah to [the World of] Asiyah.

Hence the [infinite] Ein Sof-light which is in [the World of] Atzilut is also present in [the World of] Asiyah, and [even] in this material world, by being invested in the [Sefirah of] Malchut of [the Worlds of] Beriah, Yetzirah and Asiyah, as fully explained in the writings of R. Isaac Luria, of blessed memory. (37)

25. Parenthesis in Original Text.
34. Zechariah 14:9.
35. See Tikkunei Zohar, Tikkun XXII (66b).
36. In the original, the names of the Four Worlds (in descending order) are often telescoped into the acronym, pronounced ABiYA.
37. See Etz Chayim, Shaar 42, sec. 4; et al.