Baal HaSulam, The Study of the Ten Sefirot, Part 1,“Table of Answers for the Meaning of the Words,” Question 47: What is Kav (line)? Kav indicates a discernment of up and down that was not there before. It also designates a much frailer He’arah (emanation) than before.
“A line” is a feeling of constraint. I feel lesser than someone who is above me. Besides, a line is the restriction of my receiving, according to “size” and “weight.” That is, I am limited by two conditions: the qualitative height of the degree I am at and its quantitative value that I can uncover and accept.
It’s a feeling that above me, there is another whole staircase, higher degrees and more exalted properties. On the other hand, I feel that I do not receive infinitely on every side, but am limited and can communicate with the Creator only in the form of a direct channel, a tube extending from the top down, which is called a “line.”
In other words, we should visualize not a geometric line, but rather the restrictions that the desire to enjoy experiences. All the concepts have to be translated into feelings. But there is no emotion in the shape of a line. A line is an agreed upon signal of what feeling I am supposed to evoke in myself.
Suppose I work at a factory or in an organization. And in my position on the job and in the place that I am in, I am under the whole hierarchy of chiefs more essential than I. And so, I feel beneath them in properties. Moreover, there is a difference in their ability to act or get all sorts of benefits and salary (fulfillment) at work, meaning that relative to them, I am limited in quantitative terms. Therefore, I feel vertical restriction, depending on the height (from upward to downward), as well as a quantitative restriction in this very place (horizontally).
A line indicates a discernment of up and down that was not there before. That is, I feel what it is to be “above” because I have “recollections” of them, the informational data (Reshimot). And so, I can compare it with my current state. I understand the difference between the line above and my position which I currently feel. This yields a restriction on height, that is, with respect to the Creator and compared with His properties.
It also designates a much frailer emanation than before. After all, previously, he was receiving with no limitations at the top (the head) of the line, whereas now, at its end or in the middle, he is restricted in reception.
These are two totally different experiences. The feeling that you are at the bottom of the line is due to the lack of adhesion with the Creator. A sense of how small my ability to receive is due to the lack of the ability to bestow. This means that one limitation is qualitative and the other quantitative.
From the 3rd part of the Daily Kabbalah Lesson 7/7/2011, Talmud Eser Sefirot