In Islamic jurisprudence and the pertinent laws, evidence (two fair witnesses) is considered as one of the valid statements for substantiating a claim and a verdict. In view of the jurists, evidence is recognized as a presumption validated by the legislator. Generally, the jurists and the theoreticians at law (followers of the obstruction theory and non-obstruction theory) consider presumption as a proof in view of making an acceptable conjecture, but at the same time, the majority of these experts validate evidence – that is applicable to presumption – not only in view of conjecture, but also in view of validation by legislator. They have brought the resulting conjecture (due to validation by legislator) in par with such terms as religious knowledge, common law, knowledge of certain religious issues, knowledge of interpretation, etc. However, some believe in causality and instrumentality of evidence and while accepting the absolute instrumentality of presumption, consider evidence as a proof of verdict even if the presumption is not strongly reliable. In addition to this group, some other jurists like Imam Khomeini, constitute the basis of evidence on confession by rational witnesses that has received validation and ratified by legislator. Therefore, it is the indicative nature of evidence or its instrumentality that makes it valid as a proof. The difference between the two schools of thought has laid its impact on setting requisites for validity of the evidence, its impacts and its rules such as in: obligation or no obligation for judge to make a verdict after the testimony of witnesses, validity of the number of witnesses, and the possibility or impossibility of realization of conflict of evidence. The findings of this paper support the views on instrumentality of evidence. In addition to those believing in instrumentality of evidence, those supporting causality have also claimed its applicability in such issues as successive validity, conflict of the two evidences, fairness and majority in number of the witnesses, etc.