Imam Jafar al-Sadiq, one of the Imams of the Ahl al-Bayt, Described Knowledge of Hayal (Phantom) and Wahm (Illusory) Thus

… The Imam [Imam Jafar al-Sadiq] then said, "O Hisham, how many are your senses?"

He replied, "They are five."

The Imam then asked, "Which one is the smallest?"

He replied, "The seeing sense."

The Imam then asked, "How big is its size?"

He replied, "It is about the size of the lentil or smaller."

The Imam then said, "O Hisham, look in front of you and above you and then tell me what you have seen."

Hisham then replied, "I can see the sky, the earth, the houses, the buildings, the land, the mountains and the water canals."

The Imam then said, "The One Who has been able to fit all that you can see in a lentil or even a smaller thing is also able to fit the whole world inside an egg without any change in the size of any one of them.”

Hisham bowed down and kissed the feet, the hands and the head of the Imam (a.s.) saying, "This is enough for me O descendant of the holy Prophet [saas]” and then left for his home. (Usul al Kafi, Muhammad ibn Ya'qub al-Kulayni, H 212, Ch. 1, h 4; online at http://www.al-shia.org/html/eng/books/hadith/al-kafi/part3/part3-ch1.htm)