The actual physical heart in our breast beats at about 100,000 times a day, pumping two gallons of blood per minute, 100 gallons per hour, 24 hours a day, seven days a week, 365 days a year for an entire life time! The vascular system that sends this life-giving blood is over 60,000 miles long: it is more than two times the circumference of the earth. Furthermore, it is interesting to note that the heart starts beating before the brain is formed; the heart begins to beat without any central nervous system. The dominant theory was that the central nervous system is what is controlling the entire human being from the brain, yet we know now that in fact the nervous system does not initiate the heartbeat. It is actually self-initiated; we would say, it is initiated by Allah (subhanahu wa t'aalaa).
The heart is the center of the human being. Many people think the brain is the center of consciousness, yet the Qur'aan clearly states, "They have hearts that they are not able to understand with." According to the Muslims, the center of human consciousness is the heart and not the brain itself, and it is only recently that human beings have learned there are over 40,000 neurons in the heart; in other words, there are cells in the heart that are communicating. Now, it is understood that there is two-way communication between the brain and the heart: the brain sends messages to the heart, but the heart also sends messages to the brain. The brain receives these messages from the heart, which reach the amygdala and the thalamus. The cortex receives input from the amygdala and thalamus that it processes to produce emotion; the new cortex relates to learning and reasoning. These processes are recent discoveries, and although we do not fully understand them, we do know that the heart is an extremely sophisticated organ.
According to a hadeeth, the heart is a source of knowledge. The Prophet, sallallahu 'alayhi wa sallam, said that wrong action is what irritates the heart. Thus, the heart actually knows wrong actions, and this is one of the reasons why people can do terrible things, but, ultimately, they are affected negatively.
In Crime and Punishment, the brilliant Russian author Dostoevsky's indicates that crime itself is the perpetrator's punishment because human beings have to live with the result of their actions: their souls are affected. When people do something against the heart, they act against the soul, and that actually affects human beings to the degree that they will go into a state of spiritual agitation, and people will use many ways to cover this up.
This is what kufur is: "kufur" means "covering up." To hide their agitation, people use alcohol, drugs, and sexual experimentation; they also seek power, wealth, and fame, taking themselves into a state of heedlessness, submerging themselves into the ephemeral world which causes them to forget their essential nature and to forget their hearts. Thus, people become cut off from their hearts.