Take heed, brethren, lest there be in any of you an evil heart of unbelief, in departing from the living God. Hebrews 3:12
"We think of the heart as the seat of the emotions, but the Hebrews thought of it as the core of a person—emotions, intellect, and will. The heart is the wellspring of motivation. When the author speaks of the heart believing something, he is talking about deep convictions held in the core of one’s being, the beliefs that really determine what one does. Likewise, to harden one’s heart is to make one’s will, intellect, and emotions all insensitive to God’s presence and truth."
(NavPress Bible study, Hebrews, Colorado Springs, CO: NavPress, 1989, pg 54)
This idea fits perfectly with the Book of Mormon perspective regarding the crippling effects of a hard heart. It is literally a condition that can cause spiritual blindness and deafness.
And all this have they done that they...might blind the eyes and harden the hearts of the children of men. 1 Nephi 13:27
Yea, today, if ye will hear his voice, harden not your hearts; Jacob 6:6
The condition of our hearts is the determining factor in whether we are counted among the elect of God:
...mine elect hear my voice and harden not their hearts. D&C 29:7