The basic difference between heart and mind is that the heart lives for itself.
The self is its ultimate point of reference and the ultimate objective of all its desires.
By contrast, the brain, the faculty of perception, is the power to absorb and relate to matters that are outside and beyond the self.
Indeed, the basis of all perception is the surrender of the egotistical "I" of the heart.
As long as the "I" is defined solely by the self and its own needs, it cannot assimilate any objective truth, anything that is beyond the self.
Thus, the "abode" of the Godly soul in the human body is the brain, where there is "abnegation" (bittul) of the self to another, higher reality.
The Baal HaTanya defines the ultimate kelipah (concealment of holiness) as Pharaoh's remark, "My Nile is mine, and I created myself."
When someone says, "This is mine," this is a certain degree of tumah ("profanity"); when he adds, "and I created myself," it is the ultimate profanity.
The circuit is closed completely; the kelipah is hermetically sealed.
If I made myself for myself, then the "I" is the beginning and end of all existence, and this is the ultimate descent from holiness.
In contrast, each level of holiness is a level of self-negation.
The highest level of holiness is that of the merkavah (literally, "chariot"), a state in which a person's individuality is completely negated and he operates as a vehicle, a mere tool, of the supernal will alone.
Holiness is not merely the concept that I did not create myself, but also that the "Nile" (that is, the things that are ostensibly in my domain and control) does not belong to me.
At this level, words such as "my" and "mine" are unutterable.
–Rabbi Adin Steinsaltz
From Opening the Tanya p.224, by Rabbi Adin Steinsaltz