"Undoubtedly, within the body is a piece of flesh which, when it is in good condition (salaha), the whole body is also healthy and robust (salaha); but when it is degenerated (fasada), the whole body decays. Verily, that (part of the body) is the qalb (heart)." [Bukhari]
It is apparent from the words of the hadith, the Prophet (peace be upon him) was indeed referring to the piece of flesh in the shape of the heart inside our bodies. However, his usage goes far beyond the biological meaning of the word "heart". The term qalb has a far wider meaning in the Quranic and hadith terminology. According to this understanding, our entire personality can be termed as the qalb.
When people converse amongst themselves, they normally argue based on their own personal observations and within the ambit of their own literary expression. Although science declares that the Earth revolves around the Sun, you will still say that the Sun has risen and the Sun has set. Nobody will say that the Earth has risen or the Earth has set. Likewise, the accepted idiom in our language is that my heart says so, or my heart desires such and such. This is the accepted norm for expression. Therefore, to understand its meaning, it is not necessary to determine where the biological hub of intellect lies, or what is the centre of the mind.
The human being commits sins because his qalb is not well and he errs. According to this hadith, the path of islah (reformation) is the qalb. If the qalb is rectified, the other parts of the man's body will also function properly. It will bring forth the strength to obey the shariah leading to the islah (reformation) of one's entire life. And if the qalb is in discord, then the other parts of the body will also commit wrong acts. In such a case, shariah will remain only a written law, which will not be followed; and humanity will enter into a state of disarray and discord.
This hadith addresses another important point. Amongst Muslims, we are constantly faced with the ongoing debate between shariah (rule of law) and tariqah (the Sufi path), and between the zahir (apparent) and batin (hidden). Many people consider the domains of shariah and tariqah to be different. The Prophet has combined the two into one concept through the similitude of the human body (jasad), and he has demonstrated the unity in thought. The contrasts may be perceived between the qalb and the shariah, the inner private life or the outwardly explicit one. Just as one cannot imagine the qalb without the human body, how is it possible to conceive the association of a person’s character, soul (ruh) and qalb with Allah without the establishment of salat (prayers), zakat (poor due) and fasting or without the strict observance of halal and haram? Likewise, one cannot imagine the existence of the body without the qalb. The two are portions of the same entity — belonging to the same whole and being parts of the same unity. The coupling between the two is inseparable and tied. It is not possible to distinguish between them.
"A Righteous Heart: The Axis of One's Deeds" - Khurram Murad