After the withdrawal, the empty space would have remained lifeless if nothing had entered it. Etz Chaim continues: then He drew from the light of Ein Sof "a straight line," out of His round light, from above to below, which linked downward and descended into that space. This thread is called the kav, the line. It is like, says the text, "a thin channel" through which the waters of the supernal light flow toward the worlds.
Two details change everything. The top of the kav touches Ein Sof and is attached to it; the bottom of the kav does not touch the light surrounding the space from below. Why this asymmetry? Because it is what creates above and below. If the light had entered from both ends, or from every side, there would be neither front nor back, neither east nor west, neither higher degree nor lower degree: everything would be equal again, as before the withdrawal. A single point of entry, and an order appears, a direction, a possibility of near and far. The kav is thus the first hierarchy, and all the Sefirot, all the worlds, will be arranged along it.
Etz Chaim then describes how this line, advancing slowly, forms concentric circles, then a straight figure: the igulim and the yosher, the circles and the line. The circles express an order by nearness: each circle envelops the next, and the one closest to Ein Sof is the highest. The straight line, in the image of man, adds right, left and center. One can read a lesson in this: everything is enveloped by one same presence, and yet everything is ordered by degrees.
One can relate to this thread the image of Elijah's prayer in the Tikkunei Zohar. That prayer speaks of the One who "waters the tree through that spring," like water that irrigates a tree, its arms and its branches, and makes it grow. That spring, Elijah adds, is like the soul to the body: it is its life. The kav is therefore not an object set down once and for all, but a current that keeps descending and giving life.
For your study, keep this: the kav is thin, it is measured. It is not all the light of the Infinite that enters, but exactly what the worlds can bear. The generosity of G-d is read not only in what He gives, but in the measure He takes so that the gift does not destroy the one who receives it.
