והנה, כשהאדם עוסק בתורה, אזי נשמתו שהיא נפשו האלהית, עם שני לבושיה הפנימים לבדם, שהם כח הדבור ומחשבה

When a person studies Torah, using his powers of thought and speech, his Neshamah, his divine soul with its two inner “garments” only, viz., the faculties of speech and thought*—

Thought is obviously an internal “garment”, as it is connected to the soul itself. This is why thought functions ceaselessly; it is closely bound to the soul, and the soul’s presence is constant. Speech too is internal, when compared with action, which acts upon objects outside the soul.

נכללות באור ה׳ אין סוף ברוך הוא, ומיוחדות בו ביחוד גמור

are absorbed within the light of G‑d the Ein Sof, blessed be He, and are merged within it in perfect unity.

והיא השראת השכינה על נפשו האלקית, כמאמר רז״ל שאפילו אחד שיושב ועוסק בתורה שכינה עמו

This constitutes the “resting of the Shechinah” on his divine soul; as our Sages have said:1 “When even one person applies himself diligently to Torah study, the Shechinah is with him,” meaning, in this case, that the Shechinah rests upon his divine soul, and upon its faculties of thought and speech which are engaged in the mental and oral study of the Torah.

אך כדי להמשיך אור והארת השכינה גם על גופו ונפשו הבהמית, שהיא החיונית המלובשת בגופו ממש

However, in order to draw the light and radiation of the Shechinah upon his body and animal soul as well, i.e., upon the vitalizing soul actually clothed in the body and providing for it a corporeal life-force,

צריך לקיים מצות מעשיות הנעשות על ידי הגוף ממש

one must fulfill the practical mitzvot (i.e., commandments involving the faculty of action), which are performed with the body itself.

שאז כח הגוף ממש שבעשיה זו נכלל באור ה׳ ורצונו, ומיוחד בו ביחוד גמור

In this way the actual power of the body engaged in this act e.g., when one dons Tefillin, it is the physical strength in his arm that impels the motions that constitute the fulfillment of the mitzvah; and therefore this bodily power is absorbed in the Divine light and Will, and merges with it in perfect unity.

והוא לבוש השלישי של נפש האלקית

This (power of the body) constitutes the third garment of the divine soul. As mentioned in ch.4, the divine soul expresses itself in the faculty of action through performing the mitzvot.

ואזי גם כח נפש החיונית שבגופו ממש, שמקליפת נוגה

When the faculty of action is absorbed within the Divine Will then in addition the power of the vitalizing soul actually clothed within the body, the power of that soul derived from kelipat nogah,

נתהפך מרע לטוב, ונכלל ממש בקדושה כנפש האלקית ממש

is transformed from evil to good, and is actually absorbed into holiness exactly like the divine soul.

מאחר שהוא הוא הפועל ועושה מעשה המצוה

For it is the [power of the animal soul] that implements and performs the act that constitutes the mitzvah.

True, the divine soul is the force motivating one’s observance of a mitzvah, yet the divine soul cannot directly activate the body to perform a physical action; to don the Tefillin, for example. It can accomplish this only through the agency of the animal soul, which serves as a medium between the divine soul and the body. In the Alter Rebbe’s words:

שבלעדו לא היתה נפש האלקית פועלת בגוף כלל, כי היא רוחניית והגוף גשמי וחומרי

For without this power of the animal soul the divine soul would not affect the body at all, since it is spiritual and the body physical and corporeal, so that body and divine soul are antithetical, as are the spiritual and material dimensions generally.

והממוצע ביניהם היא נפש החיונית הבהמית, המלובשת בדם האדם שבלבו וכל הגוף

The intermediary between them is the vitalizing animal soul clothed in man’s blood, which is in his heart and throughout his body.

For the nature of the animal soul is such that the coarsest, most material-like level of its spiritual substance is capable of clothing itself within the blood.

Since the animal soul is the divine soul’s medium of affecting the body, its active power is also absorbed into holiness when one performs a mitzvah.