There are various terms used in the Quran concerning knowledge and its grades. A comprehensive look at the Quranic worldview on man as well as the role and responsibility of mental faculty in having belief and faith in perceivable truths can clarify Quranic aim and purpose in its different applications of that term. A superficial look at those verses in which such key terms as ignorance, doubt, conjecture, and certainty are used may cause big mistakes in interpretation of Divine Speech in the realm of man's mental responsibilities.For in some Quranic verses doubt is considered among signs of infidelity on the one hand and in some other verses even such a personality as the Prophet is allowed to doubt what has been sent down to him on the other. Also in some Quranic verses khashi'un(the humble) are described as having conjecture about encountering their Lord, while in some other verses it is said that it is infidels who base their beliefs on conjecture.The essential question is that how can a harmonized interpretation of apparently different applications of those fundamental terms in the realm of knowledge be presented on the basis of Quranic worldview.Analyzing four significant terms in this connection, i.e., ignorance, conjecture, doubt, and certainty, this essay deals with their relation as well.