Imam Ali (peace be upon him): Allah the Exalted has not made faith obligatory upon only one limb of the body; rather, each of the limbs of the body has a duty different from the duty of the other limb... As for the faith which He has made obligatory upon the heart, it is confession, recognition, contentment with what Allah has made obligatory upon it, submission to the divine command, remembrance of Allah, reflection, and yielding to all that has come from Allah the Mighty and Majestic in the Qur'an, which must be applied to the extent of one's ability; belief in these duties is obligatory upon the heart, and one must make manifest [in action also] what he believes inwardly, except in a case where necessity dictates the concealment of belief (such as cases where taqiyya against the enemy is required), as in this saying of Allah [where He has said]: "except for those who have been placed under pressure" (Al-Nahl/106)... and this saying of Allah [where He has said]: "Do they not reflect upon the Qur'an, or are there locks upon their hearts?!" (Muhammad/24). And [again] Allah the Mighty and Majestic has said: فَإِنَّها لا تَعْمَی الْأَبْصارُ وَ لکِنْ تَعْمَی الْقُلُوبُ الَّتِی فِی الصُّدُورِ [For surely it is not the eyes that are blind, but blind are the hearts which are in the breasts]; and the like of these verses in the Qur'an is abundant, and this belief of the heart is the foundation of faith... And as for what He has made obligatory upon the two eyes, it is looking with care at the signs of Allah and averting the gaze from what Allah the Mighty and Majestic has forbidden; Allah the Exalted has said: "...Have they not looked into the dominion and system of the heavens and the earth, and what Allah has created [with care and to take a lesson]?!" (Al-A'raf/185)... and He has said: "Whoever sees with insight, it is for his own benefit, and whoever closes the eye of insight, it is to his own loss." (Al-An'am/104). And this verse encompasses both the sight of the eyes and the sight of the hearts; Allah the Exalted has said: فَإِنَّها لا تَعْمَی الْأَبْصارُ وَ لکِنْ تَعْمَی الْقُلُوبُ الَّتِی فِی الصُّدُورِ [For surely it is not the eyes that are blind, but blind are the hearts which are in the breasts].