The “resting” of the Shechinah means, i.e., when we single out a specific object from a world filled with the Shechinah by saying that the Divine Presence “rests” upon that object, its uniqueness lies in

וְהִנֵּה, עִנְיַן הַשְׁרָאַת הַשְּׁכִינָה,

the revelation of G-dliness and the Ein Sof-light in that particular object.

הוּא גִּילּוּי אֱלֹהוּתוֹ יִתְבָּרֵךְ וְאוֹר אֵין־סוֹף בָּרוּךְ־הוּא בְּאֵיזֶה דָבָר,

This means to say that this object is comprised within G-d’s light and is nullified out of existence toward Him (i.e., it has no separate identity),

וְהַיְינוּ לוֹמַר, שֶׁאוֹתוֹ דָבָר נִכְלָל בְּאוֹר ה' וּבָטֵל לוֹ בִּמְצִיאוּת לְגַמְרֵי,

for [only] then can the One G-d rest upon it (upon that object) and reveal Himself in it.

שֶׁאָז הוּא שֶׁשּׁוֹרֶה וּמִתְגַּלֶּה בּוֹ ה' אֶחָד.

But anything not completely nullified out of existence toward [the G-dly light] cannot have G-d’s light rest and reveal itself within it.

אֲבָל כָּל מַה שֶּׁלֹּא בָטֵל אֵלָיו בִּמְצִיאוּת לְגַמְרֵי – אֵין אוֹר ה' שׁוֹרֶה וּמִתְגַּלֶּה בּוֹ.

Even a perfect tzaddik, who cleaves to Him with “abundant love”—a most lofty level of love and attachment to G-d—even in the case of such a tzaddik, “no thought can truly grasp Him at all.”

וְאַף צַדִּיק גָּמוּר שֶׁמִּתְדַּבֵּק בּוֹ בְּאַהֲבָה רַבָּה, הֲרֵי "לֵית מַחֲשָׁבָה תְּפִיסָא בֵיהּ" כְּלָל בֶּאֱמֶת,

For the true implication of the phrase “G-d is the true L-rd” is His Unity and Oneness, that He alone exists, and there is absolutely nothing besides Him.

כִּי אֲמִיתַּת "ה' אֱלֹהִים אֱמֶת" – הוּא יִחוּדוֹ וְאַחְדוּתוֹ, שֶׁהוּא לְבַדּוֹ הוּא וְאֶפֶס בִּלְעָדוֹ מַמָּשׁ.

As explained in ch. 20, the statement “G-d is one” means (not only that He is the sole G-d but also) that He is the only being that truly exists. All else is totally nullified before Him.

If so, this person i.e., the abovementioned tzaddik who loves [G-d] and who is a [separate] “being” in his own self-perception (as is evident from the very fact that he loves G-d) rather than a “nonentity,” no thought of his can grasp Him at all.

וְאִם כֵּן, זֶה הָאוֹהֵב, שֶׁהוּא יֵשׁ וְלֹא אֶפֶס – לֵית מַחֲשָׁבָה דִילֵיהּ תְּפִיסָא בֵיהּ כְּלָל,

Even the tzaddik cannot grasp G-d and thereby become one with Him through the faculties of his own soul; therefore, G-d’s light does not abide or manifest itself in him except through the fulfillment of the mitzvot, which constitute His actual will and wisdom without any “concealment of Countenance.” Since the inner aspect of the Divine will stands revealed only in the mitzvot, it is only by means of them that the light of the Shechinah rests upon man, not through spiritual devotion alone.

וְאֵין אוֹר ה' שׁוֹרֶה וּמִתְגַּלֶּה בּוֹ, אֶלָּא עַל יְדֵי קִיּוּם הַמִּצְוֹת, שֶׁהֵן רְצוֹנוֹ וְחָכְמָתוֹ יִתְבָּרֵךְ מַמָּשׁ – בְּלִי שׁוּם הֶסְתֵּר פָּנִים

Concerning his aforementioned statement that the Ein Sof-light reveals itself only within that which is totally nullified before G-d and absorbed within Him, the Alter Rebbe notes:

This accords with what I heard from my teacher (the Maggid of Mezritch), peace upon him, on the meaning of and the reason behind the statement of the Etz Chaim that the Ein Sof-light does not unite itself even with the World of Atzilut (Emanation) except by first vesting itself in the attribute of chochmah (wisdom).

וְכַאֲשֶׁר שָׁמַﬠְתִּי מִמּוֹרִי ﬠָלָיו הַשָּׁלוֹם, פֵּירוּשׁ וְטַﬠַם לְמַה שֶּׁכָּתוּב בְּﬠֵץ חַיִּים, שֶׁאוֹר־אֵין־סוֹף בָּרוּךְ־הוּא אֵינוֹ מִתְיַיחֵד אֲפִילוּ בְּעוֹלַם הָאֲצִילוּת אֶלָּא ﬠַל יְדֵי הִתְלַבְּשׁוּתוֹ תְּחִלָּה בִּסְפִירַת חָכְמָה,

The Maggid (a) explained the meaning of the Ein Sof’s vesting itself in chochmah and (b) provided the reason that it does not unite with Atzilut except by way of chochmah.

This is because the Ein Sof is “the true One,which means that He alone exists and there is naught besides Him, and this, in fact, is the level of chochmah.

וְהַיְינוּ מִשּׁוּם שֶׁאֵין־סוֹף בָּרוּךְ־הוּא, הוּא אֶחָד הָאֱמֶת, שֶׁהוּא לְבַדּוֹ הוּא וְאֵין זוּלָתוֹ, וְזוֹ הִיא מַדְרֵגַת הַחָכְמָה וְכוּ':

The attribute of chochmah represents the perception of G-dliness as the sole existing being. This, then, is the meaning of the Ein Sof’s “clothing” itself in the attribute of chochmah: The revelation of the Ein Sof as “One alone, with naught besides Him.” And for this reason, the Ein Sof does not unite with any world or level except by way of chochmah, for, as stated above, G-d abides (“may be united with”) only where there is no sense of self or separation from Him, only in that which is pervaded with the spirit of “He alone…and naught besides him,” and this spirit constitutes the attribute of chochmah, as stated.

(This note expresses a profound Chasidic concept; after further study of the Tanya, we will understand it more clearly.)