In the Name of Allah, the Most Gracious, the Most Merciful. Praise be to Allah, the Lord of the Worlds, and peace and blessings be upon our Master Muhammad, the Faithful and the Honest.  

In fact, the tangible certainty is based on using our five senses and their extensions (like using the telescope or the microscope). This tangible certainty is sensed by human beings and other living beings, and it has a physical existence and physical traces. Unlike it, the rational certainty has no physical existence but has traces. Take for example electricity, sensing it belongs to the rational certainty, because we cannot see it, yet we see its traces when we turn on the light or feel the heat of the electric heater.

Hence, when something has no physical existence yet has traces, the only means to sense it, is by using the brain. This brain is the most complicated organ in the universe at all. There are 140 billion cells in the gray matter in the brain, whose function is still undiscovered. Furthermore, there are 14 billion cells in the cortex where we can find the centers of memory, judgment…etc. 

Again, when something has no physical existence yet has traces, the only means to sense it is the brain. There are more than 1000 Ayaat in the Noble Quran about using the brain:

 

[ [Ya-Sin, 68] ]

Will they not then understand? Our physical status in the religion, regarding Allah Almighty, is mentioned in the following Ayah:

However, Allah created the universe to be a sign of His Existence. Therefore, we can sense Allah's Wisdom by pondering over His creations. There is outstanding beauty in everything on earth. Regarding this point, let me tell you this critical fact: Every word consists of certain letters and phonemes, but it has no sense if the signifier does not exist. Thus, the significance of the language is based on  the physical existence of the signifier the word signifies. For example, the one who has never seen the sea has no idea about the meaning of the word "sea". However, if he goes to the sea, sails in it, sees its blue deep water and tastes its salty water, the word "sea" will bring back every memory of that trip. Thus, the words will not be understood without being related to the physical existence of their signifiers.