According to Alfārābī, knowledge is divided into two parts, conceptional knowledge (tasawwur) and propositional knowledge (tasdiq). Farabi thinks that certitude, as an epistemological problem, is a subject that belongs to propositional knowledge, not conceptional knowledge. Certitude (yaqin), which is defined as a belief that the comprehension of a particular fact or case in the mind corresponds exactly to the details of its external existence, appears when we believe something in form ‘A is B’. That is to say, certitude starts with a propositional statement. The second aspect of certitude is correspondence to reality, that is to say, a proposition’s having the attribution of truth, becoming true propositional knowledge. Therefore, certitude apprears as a secondary belief based upon true propositional knowledge and the point is that the comprehensibility of such a belief with any other type of knowledge is impossible When Farabi classified the different types the propositional knowledge, he made the concept of certainty central to his analyses. Propositional knowledge is divided into two parts, certain and uncertain. Certain propositional knowledge is to know that the correspondence between mind and reality is never going to fail. According to this view, refutation propositional knowledge via sophistical argument is absolutely impossible.