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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."