"Preface to the Sulam Commentary," “Three Come Out of One, One Exists in Three,” Item 52: Thus, it has been explained that the second discernment in raising MAN de Zeir Anpin to YESHSUT is to unite the two lines of YESHSUT, right and left. They can only shine through the Masach de Hirik in Zeir Anpin (Item 39), which completes the middle line in them and determines the two lines of Bina….
There are two lines in Bina: the right line, which is Hassadim (Mercy) and the left line, which is Hochma (Wisdom). They are not able to unite. In fact, each is ready to completely occupy the desire 100% by itself. They remain in opposition to each other.
However, they cannot emanate without each other! They both come from Above, but the right line, as if, says: “How is it possible without bestowal? Isn’t bestowal the purpose of creation?” While the left line, as if, contends: “I am the foundation of creation! I am the Light of Hochma that must be revealed in it! How can it be without me?”
And so they argue with each other without the possibility of compromise or reconciliation. Yet, in their agreement lies the solution: reception for the sake of bestowal.
To facilitate agreement, a third factor is necessary. This third factor allows each one to budge from his strongly held stance and to understand the other. The solution lies in the middle. The solution is not a compromise! Rather, it is a unique and remarkable opportunity to understand one another, to clothe in one another, and to arrive at the golden agreement.
Before the middle line, everything that existed was the qualities of the Creator. Creation begins with the middle line which allows the union of bestowal and reception. This is the form in which the creature can fully participate in bestowal and can become similar to the Creator. It is like a child who unites his parents (the two opposite lines of the Creator), and makes them one family.
From the 2nd part of the Daily Kabbalah Lesson 6/8/10, "Preface to the Sulam Commentary"