Before Baal HaSulam there were two methods of correction (or of studying Kabbalah): through the mind (the method of the Ari) and through the heart (the method of Baal Shem Tov). Baal HaSulam took both of these approaches and unified them into the middle, third line, creating an effective, precise, and simple method that is suitable for the modern souls. It is the only means enabling us to attain correction.
The scientific part of Kabbalah is based on the Ari, while the inner work comes from Baal Shem Tov. Baal HaSulam wrote the book, Talmud Eser Sefirot based on The Tree of Life by the Ari, adapting it to our generation. However, if you look at his entire teaching as a complex, then the four compositions Talmud Eser Sefirot, Panim Meirot, “Preface to the Wisdom of Kabbalah,” and “Preface to the Sulam Commentary” are devoted to the "physics" of the Upper World and are based on the teaching of the Ari, while all the other articles are devoted to a person’s inner work and are based on the method of Baal Shem Tov.
The Light of Correction comes to us while studying the "physics" of the Upper World, but it only comes through the group. If the group is not organized correctly, does not study together, and does not desire to unite, then it is impossible to attract the Light. The group is the magnet that attracts the Light that Reforms. And it must have a form that makes it ready for correction. Everyone must expect it!
We must reveal two things: 1.) our evil and lack of unity, 2.) the desirable state, the state of universal connection. We must expect that we will go from the first state to the second state with the help of the force called “the Light that Reforms.” The group is obligated to understand this very clearly since otherwise what kind of Light would you be able to attract? The spiritual vessel or desire must precede the Light.
This is why all of creation was created and why it develops until the very World of Infinity: It is in order to create a need to reveal the Creator in creation. We are not required to reveal the Creator or the Light! We have to reveal the creation! We have to understand that creation was not created in order to remain a point and to live as an animal. Rather, its destiny is to become an independent individual that has freedom of will. Therefore, our desire must always come first (Kli Kodem Le Ohr).
From the 4th part of the Daily Kabbalah Lesson 8/9/10, “Introduction to the Book, Panim Meirot uMasbirot”