Certainty has degrees. These degrees are referred to in the Qur’an as "certain knowledge”, to see with the “eye of certainty" and "certain truth".
The verses 5 and 7 of Surah al-Takathur: “Nay! If you had known with certain knowledge, you should most certainly have seen Hell; then you shall most certainly see it with the eye of certainty”
"كَلَّا لَوْ تَعْلَمُونَ عِلْمَ الْيَقِينِ ... ثُمَّ لَتَرَوْنَهَا عَيْنَ الْيَقِينِ"
refer to the first and second degrees respectively and verse 95 of Surah al-Waqi’ah:
" إِنَّ هَذَا لَهُوَ حَقُّ الْيَقِينِ "
“..Most surely this is a certain truth," refers to the third degree of conviction.
‘Ilm Al-Yaqin (Certain Knowledge)
This is reasoning-based certainty. It is reached through reasoning with majors and minors. For instance, we are convinced that fire exists when we observe the smoke, or the reasoning which takes place to prove God’s existence
"سَنُرِيهِمْ آيَاتِنَا فِي الْآفَاقِ وَفِي أَنْفُسِهِمْ حَتَّى يَتَبَيَّنَ لَهُمْ أَنَّهُ الْحَقُّ"
"We will soon show them Our signs in the universe and in their own souls, until it will become quite clear to them that it is the truth”.5
‘Ayn Al-Yaqin [To See With The Eye Of Certainty]
This is conviction derived by observing the thing itself: for example, observing the fire itself.
Haqq Al-Yaqin [Certain Truth]
This kind of conviction is arrived by touching the thing itself, i.e., a connection to the thing and adopting its characteristics, for instance, entering the fire and feeling its heat or extinction in God, and dissipation in His love.
In the sanctified tradition we read:
"مَا تَقَرَّبَ اِلَيَّ عَبدِي بِشَيئٍ اَحَبَّ اِلي مِمَّا افتَرَضتَه عَلَيه وَلا يَزَالُ العَبْدُ يَتَقَرَّبُ اِليَّ بِالنَوافِلِ حَتّى اُحبَه فِاِذَا اَحبَبتُهُ كُنْتُ سَمْعَهُ الَّذِي يَسْمَعُ بِهِ وَبَصَرَه الَّذِي يُبصِرُ بِهِ وَلِسَانَهُ الَّذِي يَنطِقُ بِهِ وَيَدَهُ الَّتِي يَبطِشُ بِهَا، اِن دَعَانِي اَجَبتُه وَاِن سَأَلَنِي اَعطَيتُهُ"
“My servant does not approach Me with anything dearer to Me than what I have made incumbent on him. He continues to approach Me through supererogatory acts until I love him. When I love him, I become his hearing through which he hears, his sight through which he sees, his tongue through which he speaks and his hand through which he strikes. If he calls on me I answer him, and if he asks me I give him.”
Fayd Kashani has told the following poem regarding this issue:
You were with me. I did not consider you as I myself,
Or perhaps you were me and I didn't know;
When I departed, I recognized you,
As far as you were me, I didn't know.
Another poet has said, regarding this:
So much I thought about you, I became entirely you,
You came gradually, and I slowly departed.
5. Qur’an 41:53.