First, the light creates the desire to receive through the four stages of direct light and fills it. But the last fourth stage of desire makes a restriction, and the light goes out of all the spheres of this desire, from all its stages. And then the fourth stage begins to attract light by a special method called “line.”
The difference between a round sphere and a line is that there were no restrictions in the sphere and light could fill all the desire created by the Creator. And in the line, the desire itself determines in what form the light will come and fill it.
The circle is the filling of desire without any restrictions, and the line is the condition that the light comes only according to the desire and the desire restricts the light, leaving only a thin line from the sphere.
From this we can see that in the four stages of direct light there is no creation yet, and everything exists as created by the Creator. But then the desire to enjoy begins to manifest itself and limits the upper light, determining how the light can fill it and connect with it.
At first, all the spheres—zero, first, second, third, and fourth—were completely filled with light. But then the fourth stage felt itself receiving and opposite to the light. So it restricted its desire, and the light was gone. But since nothing is done in the spiritual by halves, the light has departed all spheres.
And now the fourth stage itself decides in what form the light will come and fill it, and decides not only for itself but also for all other spheres. And it turns out that all this empty space, all the spheres are cut only by one thin ray (line), one sector of light.
It is like a tube through which light flows, passing outside from the world of Ein Sof to the center of all spheres, inside all space, where the restriction operates.
The line is an extent of the equivalence of the desire to receive of the creation to the desire to bestow of the Creator. The line shows how much the creation is able to resemble the Creator; the more similar to the Creator the creation can be, the thicker the line, and the less similar, the thinner the line. And this determines how much light can flow through this tube. The line is a measure of a creature’s desire to become like the Creator.
From the 2nd part of the Daily Kabbalah Lesson 11/9/20, Baal HaSulam, The Study of the Ten Sefirot