The Zohar, Chapter “Lech Lecha (Go Forth),” Item 32: “And I will make you a great nation”…“And I will bless you”…By retiring from the right line that is not mingled with the left, he was rewarded with illumination of the right that is mingled with the left…
Because he retired from the illumination of the left that is not mingled with the right, he was rewarded with the promise implied in the words, “And [I will] make your name great,” which is the illumination of the left that mingled with the Hassadim on the right.
…“And you will be a blessing,” which is the illumination of the middle line.
The advancement along three lines is like walking with two legs: on one side there is the force of bestowal (the right line), and on the other, the force of reception (the left line). One uses these two forces or lines to advance, “stepping” on one and then the other. However, the direction he moves in, which also determines how he activates these two forces of right and left, is called the middle line.
Therefore, the middle line is not some kind of third force. There is no third force! The middle line is the correct combination of the left and right lines. The correct combination ensures advancement, first via the governance of the quality of judgment, and then mercy, first good, and then evil. One triggers the revelation of the evil forces, which push man from behind. The other reveals the forces that draw and beckon him forward. Man advances with the help of these two forces. They are like two reigns through which one controls and advances himself. There is an entire science on how to use them.
The wisdom of Kabbalah is called the third line, the “golden path.” It is the method of the correct use of these two forces of nature: the force of reception and the force of bestowal, plus and minus. Our entire corporeal sciences also amount to the ability of using these two natural forces: from atomic energy, which too contains “plus” and “minus,” to molecules that join and divide on the basis of positive and negative valencies. Everything is comprised of two opposite forces.
Therefore, the correct study lies in learning how to take these two natural bases (the Light and the desire, the plus and the minus), to merge them into a single whole, and to assemble whole systems up to the infinite universe which includes everything. We have to learn so that the “mechanisms” we build through this unification, such as “engine,” “refrigerator,” “air conditioner,” and so on, all work in the best and optimal way.
Essentially, the entire wisdom boils down to extracting the maximum benefit from these two forces. And we stand in the middle between them, in the so-called “middle line.”
From the 2nd part of the Daily Kabbalah Lesson 7/16/10, The Zohar