1.Definition of ‘Ilm al-Usūl
‘Ilm al-Usūl is generally defined as ”knowledge of the assembled elements in the process of the derivation of the divine law.”
Further explanation: When a jurist wishes to derive a law regarding the obligation to respond to a greeting from the following verse, “And when you are offered a greeting, respond with a greeting that is better, or return it;”1, he would make use of the (elements that state) the imperative verb indicates obligatoriness and that the prima-facie has probative force. These two assembled principles (are then employed) in the derivation of the divine law.
It is possible to critique this definition (on the basis) that it constricts the principles under discussion to being assembled. Additionally, it indicates that these principles have been assembled for the purpose of derivation. This is while we expect a definition to explain the criteria of the subject matter, which on whose basis the scholars of Ilm al-Usῡl can decide which matters should be included and which matters should be excluded (from this science). For this reason the word “assembled” is taken out of the definition and it is said “’Ilm al-Usūl is the knowledge of the elements that are employed in the means of derivation”. Yet there remains here another important issue. This definition fails to explain the criteria on which a matter is included or excluded from this science. For example, a linguistic matter such as the prima facie of the word “ground” (sa’īd) also is employed in the derivation process (yet is not part of ‘Ilm al-Usūl).
Therefore it is more worthy to define ‘Ilm al-Usῡl as “knowledge of the shared elements in the procedure of the derivation of the divine law”. The intention behind being shared is the suitability of the element in being used in the derivation of all possible instances that the jurist is vested with the responsibility of deriving the law of. So for example if we take (the element of) the imperative verb indicates obligatoriness, it is possible to employ this element in deriving the obligation of prayer as well as the obligation of fasting (i.e. it is not limited to one or a handful of matters). On this basis we can exclude matters such as the prima facie of the word “ground” from ‘Ilm al-Usūl, as this element does not have the suitability of being employed in the derivation of law in matters unrelated to it.