Logicians view both aposteriori and apriori methods of reasoning as arguments. However, the question is whether both of them enjoy equal levels of validity, and whether both attain logical certitude. Based on the principle of “the possessors”, stating that the knowledge of existents which have a cause can only be attained through the knowledge of their cause, Ibn Sina maintains that only the apriori argument, which attains perfect certitude, enjoys logical validity, while the aposteriori argument lacks validity since it does not attain logical certitude. However, he considers the aposteriori argument to be valid and a tool for attaining certainty through general concomitants. Unlike Ibn Sina, Mulla Sadra views all aposteriori arguments to be valid because, in his view, the existence of the effect certainly indicates the existence of the cause. As a result, the knowledge of the existence of the effect leads to the knowledge of the existence of the cause. This paper demonstrates that Ibn Sina’s idea in this regard is justified, while that of Mulla Sadra is illogical and unacceptable because of its inconsistency with well-established logical principles.