We may encounter circumstances in which we cannot deduce the Islam ruling (Hukm sharii) from the four sources (primary sources-daleel Sharii) .In this circumstances there is many practice principles(Usul Amaliyyah-Secondary Sources) from which we can interpret Islamic ruling. So Usul Amaliyyah are used to deduce Islamic ruling, not subjects. But sometimes, in order to prove the subject of the ruling, it is also referred to the practice principles and they called subjective principles, such as subjective presumption (Istishab mowzoei). On the assumption of subject, the actual existence of it cannot be proved; since this issue is not in the authority of the legislator (sharea’), rather, with regard to deduce Islamic ruling, that subject is supposed to be existing, here it is called Apparent law (Hukm-e Zaheri). Now, if the subject has the ordinary and rational requisites, and an Islamic ruling is established about it, with application of principle it is not possible to prove the requisites and the Islamic ruling that has been established. The proof of the rational and ordinary requisites depends on the actual existence of the subject (Mustashab). This subject is proposed in Usul Al-Fiqh under the Asl Mosbet. It is said that practice principles, do not prove their ordinary and rational requisites of their issue and they are not proving. We criticize and analyze different opinions about it and express the basics of its lack of authority.