The background of classification of sciences goes back to ancient Greece. Among the Greek philosophers, we may mention Aristotle whom some consider the first person who could classify sciences. He divided philosophy into theoretical and practical, and classified mathematics, natural sciences, public affairs and theology under the former, while considering the latter as including three sections: management of the house (domestic policy), management of cities (civil policy), and ethics. Muslim philosophers have divided theoretical philosophy into three sections. For them, theology is not an independent science and is among the pubic affairs. The justification and explanation of this classification, considering the fact that they have specified ‘subject’ as the criterion, is thought-provoking. The most important provision in explaining the subject is ‘essential accident’. In this study, we have used a descriptive-analytical method to state the views offered by Mohammad Reza Mozaffar and Morteza Motahhari regarding the essential accident and refer to the implications of this idea. The results are as follows: according to Mohammad Reza Mozaffar’s view of essential accident, all sciences are philosophical, but Morteza Motahhari’s view does not have such a drawback, and all propositions of sciences are in their special place.