Shahid Baqir Sadr on the Application of the Principle of  al-Wâhid in Ilm al-Usul

The interaction between philosophy and principles of jurisprudence as two important branches of knowledge in the area of Islamic sciences can have significant outcomes and effects. The principle of ‘al-wâhid’ is one of the philosophical principles with a great many functions in juridical areas. To prove this, the most important argument is the principle of cognation (similarity) of the cause and the effect in the sense that if different things (effects) emanate from a single thing, then there must be some aspects of multiplicity in the essence of that cause because there must be cognation between cause and effect whereas, in fact, "there will not be issued from one thing more than one". In other words, insofar as something is considered to be one thing, it can have no multiple dimensions.
The principle of ‘al-wâhid’ has numerous applications in Ilm al-Usul. The present study seeks to analyze ten applications of this principle in juridical areas according to Shahid Sadr. First application: Proving a subject for every science; second application: The use of a word for more than one meanings; third application: The assumption that a word with an absolute and unrestricted meaning is used for a particular thing; fourth application: The assumption that a word includes all correct instances in the issue of sahih and a’amm; fifth application: the assumption that a word includes all correct and incorrect (fasid) instances in the issue of sahih and a’amm; sixth application: Analyzing wujūb takhyiri (optional obligatory duty) in the state of contradictions; seventh application: Analyzing wujūb takhyiri in minimal and maximal quantity; eighth application: Definition of concept; ninth application: Proving a concept; Tenth application: The issue tajarri .