Full Knowledge of Allah’s Essence is Impossible

It is impossible to describe Allah, the Exalted, with an attribute which encompasses His Holy Essence. Among those who asked the Imām about this matter was Abū Hishām al-Ja‘fari, who was one of the greatest figures of his time in virtue, knowledge, and Allah-fearingness. He asked the Imām the following question:
“Can Allah be described (i.e., specified in words)?”
Through this question, Abū Hāshim al-Ja‘fari wanted to find and to encompass the knowledge of Allah, the Most High, so Imām al- Rida, peace be on him, asked him:
“Do you not read the Qur’ān?”
“Yes,” he replied.
The Imām answered through the Book of Allah, the Most High, saying:
“Do you not read these words of Him, the Exalted: The eyes attain him not, but He attains the eyes?”
“Yes,” was the answer.
“Do you know (the meaning of) the eyes?” asked the Imām.
“Yes,” came the answer.
“What is it?” asked the Imām.
“It is the seeing with the eyes,” replied Abū Hāshim.
The Imām, peace be on him, disproved this answer, saying: “Verily, the conceptualization of the heart is far greater (embracing in knowledge) than the vision of the eye. Still the conceptualization of the heart cannot attain Him, whereas all conceptualization is in His grasp.”
Certainly, the conceptualization of reason is far greater and more inclusive than the vision of the eye. Reason is also limited in conceptualization and imaginations, hence it cannot encompass the Essence of Allah, the Most High. ( Usūl al-Kāfi, vol. 1, p. 99.) 

Reference :The life of Imām'Ali Bin Mūsā al-Ridā