Baal HaSulam, Letter #8, “Fruit of Wisdom”: “They say that there is a sublime purpose for all that happens in this world, and it is called “the drop of unification.” When those dwellers of clay houses go through all those terrors, through all that totality, in His pride, which is removed from them, a vent opens in the walls of their hearts, which are tightly sealed by the nature of Creation itself, and they become fit for instilling that drop of unification in their hearts.”
Question: The Light is whole when it is revealed, so is it called only a “drop of unification?”
Answer: You need no more than a point of connection in order to attain unification. It is like the birth of a new life, in which the father gives the whiteness, which is merely a drop. In fact life is created from one sperm cell, from one point of life, and there is no need for anything more than that. The drop of unification comes from the Creator, it is the photon, that particle of the Light, and there is no need for anything but that. Everything else comes from the created being, from his readiness, and then he reaches complete adhesion.
From the Preparation to the Daily Kabbalah Lesson 3/11/14