Understanding the essence of knowledge and perception and the manner of acquiring perceptions in general, and rational perceptions in particular has had a special place in Moslem philosophers’ thought from antiquity. Mulla Sadra is among Moslem philosophers who have extensively and differently dealt with this issue. This article first explains the basic principles of Mulla Sadra’s thought about knowledge and then explicates three seemingly different theories of his concerning the quality of acquiring rational perceptions or intellection. Finally, there is a synthesis of these three theories and its reference to different transformations of the soul.