ʿAyn al-yaqīn (the eye of certainty) is reserved for those who possess knowledge, while ḥaqq al-yaqīn (the truth of certainty) is reserved for those who possess maʿrifa (gnosis, experiential knowledge of God). (Qushayrī, 47)
Knowledge for which there is no yaqīn (certainty) regarding its soundness is not knowledge. When knowledge is attained, the absence (ghayba) within it becomes as if it is seen with the eye. For, just as the believers (muʾminūn) today know Allah by a certain attribute, tomorrow they will see Him with that same attribute. If, on the contrary, they were to see Him differently, it would mean that either their vision tomorrow is unsound or their knowledge today is incorrect. Both cases represent two different aspects of opposition regarding tawḥid (the oneness of God). For the knowledge that people (al-khalq) have about Him today is sound, and their seeing Him in the Hereafter tomorrow is also sound. Now, ʿilm al-yaqīn (the knowledge of certainty) is like ʿayn al-yaqīn (the eye of certainty). Ḥaqq al-yaqīn is also like ʿilm al-yaqīn. The view of those who say that knowledge will disappear in ruʾya (the beatific vision) is invalid, for ruʾya is an instrument for the attainment of knowledge, just like hearing, listening, and other senses. As it is impossible for knowledge to disappear in the ear, so too is it impossible for it to disappear in the eye.
What the Sufis mean here by ʿilm al-yaqīn is knowledge regarding the rulings of divine commands and worldly transactions. What they mean by ʿayn al-yaqīn is knowledge concerning the state of agony at the time of departing from the world (dunyā). What they mean by ḥaqq al-yaqīn is knowledge regarding the unveiling of the vision in Paradise and the witnessing of the states there with the eye. ʿIlm al-yaqīn, by the judgment of uprightness (istiqāma) of the scholars, pertains to the rulings of affairs related to the sharia (the sacred law). ʿAyn al-yaqīn, by the judgment of being prepared for death, is the maqām (spiritual station) of the gnostics (ʿārifūn). Ḥaqq al-yaqīn, by the judgment of turning away from all created things, is the locus of fanāʾ (annihilation in God) for the friends (awliyāʾ) of God. (Hujwīrī, 464)
Allah the Exalted has said: "Indeed, in the earth are signs for those who possess yaqīn (certainty)." (al-Dhāriyāt 51:20) Yaqīn is the mount of the one who enters this path, the end of the degrees of the common people, and it is said to be the first step of the elect.
There are three degrees of yaqīn. The first degree is ʿilm al-yaqīn, which is to accept everything that appears from the Truth (al-Ḥaqq) and is absent regarding the Truth, and to be aware of everything that subsists by the Truth.
The second degree is ʿayn al-yaqīn, which, by the way of istidrāk (disclosure of hidden matters)...