The Zohar, Chapter "VaYetze (And Jacob Went Out)," Item 60: … Indeed, so it is in the Merkava—Abraham and Isaac are right and left, and Jacob is in the middle. It is written, “The land,” Nukva, “On which you lie.” Thus, together, they are all one holy Merkava, the three lines with the Nukva. And here Jacob saw that he would be the senior among the patriarchs.
The Zohar tells us so much about the lines and especially about the ones that are opposite one another (about Abraham and Isaac) since we simply don’t sense yet how hard it is for a person to work with intention and action that are opposite one another. After all, in our world, everything works in one direction. If I give to someone with the intention to do something good for myself, it doesn’t mean that I give and that I work in two directions. It is work in a single direction which is for myself.
When we begin to work outside of the desire to receive pleasure, then every element includes two opposites which are in intention or in action. Hence, it’s not a coincidence that The Zohar always encounters such calculations. It seemingly says over and over where Abraham is, where Isaac is, and where Jacob is, or in other words, where the right line and the left line are.
This is because the work in three lines is very difficult on each level. It is the connection of the desire to receive pleasure that doesn’t have anything to do with spirituality, or with the intention to bestow so that this exact desire would become active, determining, building the foundation, and performing a spiritual act.
From the 1st part of the Daily Kabbalah Lesson 6/14/10, The Zohar