Binah, or understanding, is the third of the ten sefirot, and the second conscious power of intellect in Creation. Binah appears in the configuration of the sefirot at the top of the left axis, and corresponds in the tzelem Elokim to the left hemisphere of the brain. In its fully articulated form, binah possesses two partzufim: the higher of these is referred to as Imma Ila'ah ("the higher mother"), whereas the lower is referred to as Tevunah ("comprehension"). These two partzufim are referred to jointly as Imma ("the mother").
Binah is associated in the soul with the power of conceptual analysis and reasoning, both inductive and deductive. The partzuf of Imma Ila'ah is associated in particular with the power to grasp and comprehend the insights of chochmah, whereas Tevunah represents the power to fully assimilate the resultant ideas into one's consciousness.
The "understanding" of binah also implies the ability to examine the degree of truth or falsehood inherent in a particular idea. This is expressed in Job as (12:11 and 34:3): "the ear examines words." The ear, the sense of hearing, is associated with binah.
"Hear, O Israel..." (Deuteronomy 6:4) means "Understand...." The initial letters of the phrase "the ear examines words" spell emet, "truth."
Another feature identified with the property of binah is the ability to explain and elucidate concepts both to oneself and others. For this reason it is symbolized in Kabbalah as "the wide river."
The word binah derives from the root bein which means "between." The power of binah is to distinguish and differentiate between ideas. Binah itself is the second "brain," between chochmah and da'at.
The union of chochmah and binah ("the higher union," in Kabbalah), the "father" and the "mother" (the right and left hemispheres of the brain) is continual, and is referred to in the Zohar as "two companions that never separate." This union is necessary for the continual recreation of the world (beginning with the birth of the seven attributes of the heart, corresponding to the seven days of Creation, from the womb of "mother," binah).
The union of chochmah (73) and binah (67) = 140. 140 is the sum of all square numbers from 1 to 7. This reflects the source of all the 7 attributes of the heart (days of Creation), the 7 "children," in their ultimate state of perfection (a square number represents a perfected state of being) in the mind of "father" and "mother."