In man, Allah Ta’ala has created human heart as the best and chief Organ and the Qur’ân labels it as Qalb. As Rasulullah (Sallallahu Alaihi Wasallam) says:
ألا إنّ حِمَى اللَّهِ في أرْضِهِ مَحَارِمُهُ ألا وإنّ في الجَسَدِ مُضْغَةً إذا صَلَحَتْ صَلَحَ الجَسدُ كُلُّهُ وإذا فَسَدَتْ فَسَدَ الجَسَدُ كُلُّهُ ألا وهْيَ القَلبُ
"Indeed there is a piece of flesh in the human body, if it stays alright, the whole body stays alright, if it goes astray, the whole body goes astray. Listen with open ears: that is Qalb”. (Bukhâri Sharief)
What is Qalb
It is necessary to mention here that the word Qalb is not synonymous with the human heart which consists of muscular and connective tissues filled with some venous and arterial blood that we see in the left side of the chest, but as per the definition given in "Jalalain" (an authentic commentary of the Holy Qur’ân): Qalb is an instrument which has been given the capability of perceiving the divine Attributes.
القلب وهو مشرق اللطيفة الإنسانية و يطلق على نفس اللطيفة النورانية الربانية العالمة التي هي مهبطة الأنوار إلهية الصمدانية وبها يكون الإنسان إنساناً و بها يستعد الكتاب الأوامر و اجتناب الزواجر و هي خلاصة تولدت من الروح الروحاني و يعبر عنها الحكيم بالنفس الناطقة و لكونها هدف سهام القهر و الطف و المظهر الجمال و منشأ البسط و القبض و مبدأ المحو و الصحو و منبع الأخلاق المرضية و الأحوال الردية و كثير من الناس ذهب إلا أن تلك مضغة هي محل علم و قيل أنه في الدماغ و قيل أنه مشترك بينهما و بني ذالك على إثبات الحواس الباطنية و الكلام فيها مشهور و من راجع وجدانه أدرك أن بين الدماغ و القلب رابطة معنوية و مراجعة سرية لا ينكرها من كان له قلب أو ألقى السمع و هو شهيد (روح المعاني)
Allah Ta’ala says in the Qur’ân:
فَمَنْ يُرِدِ اللَّهُ أَنْ يَهْدِيَهُ يَشْرَحْ صَدْرَهُ لِلْإِسْلَامِ
‘Those whom He willeth to guide - He openeth their breast to ‘Islam”. (6:125)
This opening up of the breast, which is the seat of the Qalb, has been linked with understanding of guidance from Allah Ta’ala .
Rasulullah (Sallallahu Alaihi Wasallam) says in a Hadith:
"My eyes sleep, my heart does not sleep”. (Mishkat Sharief)
Rasulullah further says in a Hadith:
"lndeed, a Ghain (a sort of veil) is put on my heart and indeed I ask Istigfar (repentance) to Allah Ta’ala 100 times a day." (Muslim Sharief)It is because of this Istigfar that these veils were getting lifted and the Qalb-i-Mubarak (blessed heart) of Rasulullah (Sallallahu Alaihi Wasallam) continued to receive higher and higher states of the Divine Knowledge.
Shah Wali-Ullah Muhaddith Delhvi (RA) says:
"Qalb is that thing by which man loves and hates."
This means according to Shah Sahib Qalb is the seat of love and hatred.
In other words we have different instruments for knowing different things, for example, we have thermometer to help us know the temperature of a human body, sphygmomanometer is there to tell us the blood pressure of a person. So what instrument Allah Ta’ala has created in this universe with which one can perceive Him, get His Ma’ârifat and has the capability of receiving and perceiving the rays of the Attributes of Allah Ta’ala. You have got the answer that it is the Qalb. Since it is closely associated with the human heart so the two are usually thought to be synonymous and both words are usually used for describing the same thing, though there is a difference as already explained. It is in view of this greatness and extraordinary important function of the human heart that Rasulullah (Sallallahu Alaihi Wasallam) once caught the edge of the outer curtain of Khana-Ka’aba and said:
“O Khanah Ka‘abah you are indeed great but the Qalb (Heart) of a Mumin (True believer) is greater than you" (Ibn Majah Sharief)
In this Hadith, Rasulullah (Sallallahu Alaihi Wasallam) is referring to the greatness of the human heart. This greatness of the human heart is only because of its most important function i.e., Ma’ârifat-i-Ilâhiyah. It is because of this function which makes it king of the rest of the organs (Ra’eesul Aadha).
Allah Ta’ala confirms this function of heart or Qalb by saying in the Qur’ân:
أَفَلَمْ يَسِيرُوا فِي الْأَرْضِ فَتَكُونَ لَهُمْ قُلُوبٌ يَعْقِلُونَ بِهَا أَوْ آذَانٌ يَسْمَعُونَ بِهَا فَإِنَّهَا لَا تَعْمَى الْأَبْصَارُ وَلَكِنْ تَعْمَى الْقُلُوبُ الَّتِي فِي الصُّدُورِ
"Do they not travel through the land, so that their hearts (and minds) may thus learn wisdom and their ears may thus learn to hear. Truly it is not the eyes that are blind but the hearts which are in their breasts.” (22:46)
This statement of the Qur’ân clearly reveals that it is only the heart that can perceive the Reality. These external eyes, which have a limited range of vision, cannot see the Reality. These can only see the coarse things and not the benign things. The range of the vision of heart i.e., inner vision, is infinite. The range of the vision of external eyes is no comparison to it.