Baal HaSulam is talking about two lines. The left line is the desire; the Klipa (shell) is the will to receive in order to receive. In the Kedusha (sanctity) it’s a desire which, to begin with, comes with the intention to receive. Then a person turns to the right, to the quality of bestowal, to the Light, and balances these two lines within him with BehinatDaat. As we say Hochma, Bina, Daat, which is the side of the Gevura from the left, and the side of the Hesed or the Hochma from the right, and that is the way we build the lines...