Tabatabai, 1 in his explanation of the term "al-Ghayb" (the unseen), says that it is the opposite of "the perceived", and that we cannot perceive through our usual senses. He adds that the term is used to denote Allah, and His great signs, including the revelation, which is referred to in the Qur'an 2: 4; And who believe in the Revelation sent to thee, and sent before thy time, and (in their hearts) have the assurance of the Hereafter. 2 He affirms that it also includes the hereafter, but in this verse, the belief in the revelation and in the hereafter have been mentioned separately. Therefore, "the unseen" must have been used only to mean Allah. In this way the belief in the three fundamentals of religion becomes complete.
Tabatabai then reports from the Ma'ani al-akhbar a tradition from Imam al-Sadiq (sixth Imam), who said that: "Who believe in the Unseen" means those who believe in the rising of al- Qa'im (the Mahdi) and consider it as the Truth."
1 Al-Mizan: Vol.1, p.68
2 æ ÇáÐíä íÄãäæä ÈãÇ ÇäÒá Çáí˜ æ ãÇ ÇäÒá ãä ÞÈá˜ æ ÈÇáÂÎÑÉ åã íæÞäæä.
The author comments that this explanation is given in other traditions also. But Tabataba'i adds that in order to interpret the verse as concerning the Mahdi in this case implies transforming the general meaning of the verse to a specific one, the verse having a general meaning.