Tanya for the 8th of Cheshvan ח' מר חשון

As the laws become vested within lower realms, they can become subject to a measure of concealment. Likewise, as the sefirah of malchut of Atzilut becomes vested in lower Worlds, it too is subject to this state of concealment. It is then called the Tree of Knowledge of Good and Evil because it is vested within kelipat nogah. This vestiture takes place for the sake of one of the ultimate spiritual tasks of man—beirurim, i.e., sifting and refining the physicality of this world, in order to elevate the Divine sparks from the evil which encumbers them.