The study attempts to prove the jurisprudents justification of rulings by necessity, and considering them as a path of deduction and ijtihad. The study consists of two sections. The first topic: It came in three demands that dealt with the first requirement: the definition of the study terms, which are the cause and the need in language and terminology. As for the second requirement: it dealt with the theorizing aspect by reasoning with need, and its relationship with the term wisdom, and whether need means wisdom, or does it fall under it, or is there a heterogeneity between them, and the difference of jurists in reasoning with wisdom, and inferring the validity of the reasoning with the need in practice through the presentation of the sayings of the jurists. As for the third requirement: it includes a statement of the controls of need and the conditions governing it. The second topic dealt with the practical aspect, which is the application of the need in financial transactions and the jurisprudence of jurists in it, and the reason for choosing this field of application, because financial transactions are close to people, and they are always in renewal and expansion with the changes of time and place and their requirements. This study came in four demands: It talks about the tools of need in its work and the realization of its purpose, which are: text, custom or dealing, appendix, dependency, and interest, to emphasize that the jurists justified the need if its conditions and tools are available, and that it is originally in the legislation of contracts, whenever people need a sale or contract, it expands It has nothing else. In the conclusion, a presentation of the most important results reached by the researcher, with a list of the most important sources and references that the researcher relied on in his study.