Up to now the Alter Rebbe has discussed the mitzvah of loving one's fellow, on its own merits. He now proceeds to discuss the value of this mitzvah as the basis for all the commandments, thereby elucidating yet further the importance of "rejoicing with the joy of the soul alone."
The Talmud relates that it was Hillel the Elder who authored the well-known statement that ahavat Yisrael (the love of one's fellow Jew) is the basis of the entire Torah. For Hillel had been approached by a gentile who declared that he wished to convert to Judaism, but only if Hillel would teach him the entire Torah while he stood on one foot. Hillel replied, "What is hateful to you, do not do to your fellow. That is the whole Torah; the rest is but commentary...
An obvious difficulty presents itself. All mitzvot fall into either of two categories: (a) bein adam lachaveiro — "between man and man," and (b) bein adam laMakom — "between man and G‑d."
It is readily understood how all the mitzvot of the former category may be motivated by one's love of his fellow. But how can this love motivate one to fulfill any of the mitzvot belonging to the latter category — to observe the Sabbath, for example
The Alter Rebbe's answer follows from his previously stated principle that the essence of ahavat Yisrael lies in giving priority to one's soul rather than to his body. This indeed is the basis of the entire Torah — as the Alter Rebbe continues:
וזהו שאמר הלל הזקן על קיום מצוה זו: זהו כל התורה כולה, ואידך פירושא הוא וכו׳
This explains Hillel the Elder's statement concerning the fulfillment of this mitzvah:1 "This is the entire Torah, the rest is but commentary."
כי יסוד ושורש כל התורה הוא להגביה ולהעלות הנפש על הגוף מעלה מעלה עד עיקרא ושרשא דכל עלמין
For the basis and root purpose of the entire Torah is to elevate and exalt the soul high above the body, to [G‑d], the root and source of all worlds,
וגם להמשיך אור אין סוף ברוך הוא בכנסת ישראל, כמו שכתוב לקמן, דהיינו, במקור נשמות כל ישראל, למהוי אחד באחד דוקא
and also to draw down the infinite light of Ein Sof into the Community of Israel — as will be explained further, 2 meaning into the fountainhead of the souls of all Israel, so that "the One [G‑d] will reside within [Israel — but only insofar as they are] one," i.e., united.
ולא כשיש פירוד חס ושלום בנשמות, דקודשא בריך הוא לא שריא באתר פגים
But this indwelling of the light of Ein Sof in the Community of Israel is impossible if there is disunity between the souls, G‑d forbid, for "G‑d does not dwell in an imperfect, fragmented, place." 3
וכמו שכתוב: ברכנו אבינו כולנו כאחד באור פניך, וכמו שכתוב במקום אחר באריכות
So do we say in our prayers: 4 "Bless us, our Father, all as one with the light of Your Countenance," indicating that "the light of G‑d's Countenance" can be revealed only when we are united "all as one," as explained elsewhere at length.
Since every Jew has a divine soul, and since the commandment to love one's fellow is based on the essential unity of the souls, it follows that this commandment applies to every Jew without exception. In fact, however, we find the Talmud exhorting us to hate certain fellow Jews. How do we reconcile these apparently contradictory requirements
The Alter Rebbe proceeds to clarify:
