In ch. 31, the Alter Rebbe discussed various means of arousing joy to counteract the sadness brought on by contemplation of one’s spiritual failings. Ch. 33 resumes this discussion.

עוד זאת תהיה שמחת הנפש האמיתית, ובפרט כשרואה בנפשו בעתים מזומנים שצריך לזככה ולהאירה בשמחת לבב

Yet another means of leading one’s soul to true joy, especially at those specific times when one finds it necessary to purify his soul and illuminate it with a gladness of heart:

אזי יעמיק מחשבתו ויצייר בשכלו ובינתו ענין יחודו יתברך האמיתי

Let him then think deeply and picture in his intellect and understanding the subject of G‑d’s true unity.

True unity means not only that there is but one G‑d, one Creator, but that furthermore, G‑d is the only existing being — nothing truly exists outside of Him, as will be explained further.

איך הוא ממלא כל עלמין עליונים ותחתונים

Let him consider how He permeates all worlds, both upper and lower.

Just as the soul pervades the body, thereby animating it, so does G‑d permeate all the worlds. This indwelling refers to the divine life-force which adapts itself to each individual creation’s capacity to receive it, and for this reason the Alter Rebbe distinguishes here between the “upper worlds” and “lower worlds”: in the “upper (more spiritual) worlds” the revelation of this life-force is greater, since their capacity is greater.

ואפילו מלא כל הארץ הלזו הוא כבודו יתברך

Let him further consider how even this world is filled with His glory —

This refers to the divine life-force which “encompasses” all worlds, and which animates them as if “from above,” without adapting itself to the particular nature of each created being, so that even this physical world is “filled with His glory”1 —

וכולא קמיה כלא חשיב ממש

and how everything is of no reality whatever in His presence.

והוא לבדו הוא בעליונים ותחתונים ממש כמו שהיה לבדו קודם ששת ימי בראשית, וגם במקום הזה שנברא בו עולם הזה, השמים והארץ וכל צבאם, היה הוא לבדו ממלא המקום הזה

He is One alone in the upper and lower realms, just as He was alone prior to the six days of Creation, when nothing existed apart from G‑d; so too now, when all the worlds have come into being, He is still One alone since all of creation is naught before Him, as will be explained further. Even in the very place where this world — the heaven, the earth and all their host — was created, He alone then filled this space.

וגם עתה כן הוא לבדו בלי שום שינוי כלל, מפני שכל הנבראים בטלים אצלו במציאות ממש

The same is true now; He is One alone, without any change whatever. For in relation to Him, the very existence of all created beings is utterly nullified — so that from His perspective, as it were, everything remains just as it was prior to creation.