When we are in a circle, it means that we remain in the general system of nature, which includes the entire still, vegetative, and animate nature and the human souls in the system of Infinity.
A line is when we take on the function of bringing this system into action instead of letting it be launched from the above, by nature.
A circle represents the condition of absolute mutual guarantee from the above, and a line represents the extent of our ability to realize the mutual guarantee on our behalf.
When we observe the law of mutual guarantee, the still, vegetative, and animate nature also join this process because it is included in our “human” degree. This way the entire system on all of its levels participates in the mutual guarantee.
From the 4th part of the Daily Kabbalah Lesson 10/15/10, Writings of Baal HaSulam