Shaar HaYichud VehaEmunah1

The theme of this treatise, as the Rebbe notes, is stated in its subtitle:

להבין מעט מזעיר מה שכתוב בזהר, דשמע ישראל כו׳ הוא יחודא עילאה, וברוך שם כבוד מלכותו לעולם ועד הוא יחודא תתאה

Let us understand [at least] in a small measure, the statement of the Zohar,2 that3 Shema Yisrael... is yichuda ila‘ah (“higher-level Unity”) and4 Baruch shem kvod malchuto leolam vaed is yichuda tata’ah (“lower-level Unity”). For vaed equals echad through the substitution (and thereby the descent) of letters,5 as stated in the Zohar.

Thus, the object of Shaar HaYichud VehaEmunah will be to understand how it is possible to speak of two different levels of Divine Unity.