Free translation from a talk of the Rebbe, Shabbat Parshat Vayeishev, 5744 (1983) (excerpt)

We have spoken of both Ahavas Yisroel (love of a fellow Jew) and Achdus Yisroel (unity of Jews). Some people have asked: Scripture says only “You shall love your fellow as yourself.” What is the source for the idea of Achdus Yisroel?

The source is in the Talmud Yerushalmi (Nedarim 9:4). It explains the command “You shall love your fellow as yourself” with a parable to “one who cuts meat with a knife and cut his hand. Would it enter your mind to in turn cut the hand which wielded the knife (which cut the other hand)? So too Torah says, ‘You shall love your fellow as yourself.’“

The command “You shall love your fellow as yourself” seems quite self-evident. Why does the Talmud Yerushalmi find it necessary to give a parable, and how does this parable clarify the idea of Ahavas Yisroel?

This parable explains the reason for loving another Jew as oneself. Jews are as limbs of one body, as illustrated in the parable of the person cutting his hand — should one hand cut the other? This is the idea of unity of Jews: not only should Jews love each other (which can be possible even while remaining separate entities) but they are one body.

But if Achdus Yisroel is the reason and cause for Ahavas Yisroel, then the question is reversed: the Torah should have commanded Jews regarding Achdus Yisroel, and Ahavas Yisroel would automatically follow. Yet Torah says, “You shall love your fellow as yourself.”

However, in Torah and mitzvos, principal emphasis is always placed on practical deed, and not on the causes which lead to the deed (feelings and thoughts). In teshuvah (repentance) for example, the mitzvah is to actually confess one’s transgressions; it is not the regret one feels in the heart — although “the principal idea of teshuvah is in the heart.”

So too Torah in general: It deals mainly with the things a person must do and must not do, and not so much with the reasons for them: “we will do” rather than “we will hear.” In Torah study, too, a child first learns about practical deed and not the basic philosophical tenets of faith.

Ahavas Yisroel and Achdus Yisroel are no exception. The former affects actual deed, how to behave with one’s fellow. The latter has nothing to do with deed, but rather with one’s feelings and understanding. Thus, because Torah concentrates primarily on deed, only the command to love one’s fellow is given.