The concept of taʿlīl occupies a central position in the Arabic intellectual tradition. In grammar it covers a host of areas of immense theoretical interest, including description, methodology, epistemology and explanation. This book sets out to deal with the concept by investigating the major works of those grammarians who have contributed most in theoretical terms to its development and elaboration in the Arabic Grammatical Tradition. It seeks to do so by considering the relation between grammar, on the one hand, and jurisprudence and theology, on the other. In dealing with the above issues an appeal is made, both directly and indirectly, to some of the relevant ideas in the philosophy of science as they apply to this tradition. The approach adopted is cross-disciplinary in orientation to reflect the centrality of taclil in the Arabic intellectual tradition, and reference is therefore made to the employment of this concept in jurisprudence and, to a lesser extent, theology. This is the first major study in any European language or in Arabic to be published on the topic. It will be of interest to Arabic grammarians, students of Arabic and general linguistics and to specialists in Islamic jurisprudence and philosophy.