Background and Purpose: The concept of cause and condition have been used in various fields of economic law, including sales. In order for the necessary and appropriate to occur, it must be connected with the absence of obstacles in the world of evidence. In the world of proof and in legal events, it is impossible to discover all the obstacles, therefore, if there is doubt about the existence of an obstacle, we act on the occurrence and existence of the appropriate.
Materials and Methods: This research was carried out using library sources in descriptive-analytical method.
Ethical Considerations: In this research, the principle of trustworthiness and originality of the work have been respected.
Findings: Approaching the cause of any legal phenomenon and discovering the components of its root cause and explaining the role of those components and their angles and fields of application in legal actions and events and their correct management is the solution of this rule.
Conclusion: If the dogma of the necessary existence for the occurrence or survival of a legal phenomenon is formed in human beings and is doubted in the presence of obstacles, then this rule rules the occurrence or survival of the necessary according to the principle of Istihab. Many of the rules of civil law, such as the necessity and validity of contracts, are based on the rule of "necessity and hindrance".