In his work, Ṭūsī attempts to modify the radically rationalist and pragmatic positions of al-Murtad̓ā (positions already present in embryonic form in the work of al-Mufid): rehabilitation of the first traditionists, ilm-ul-hadith attested by a single authority so long as these are conveyed by reliable sources and conditional validity of traditions conveyed by transmitters professing “deviant” doctrines. In politics, serving an unlawful government (in this instance, the ʿAbbāsid caliphate) is in certain circumstances desirable, and collaboration with a power claiming that its authority derives from the Hidden Imām (a clear reference to the Buwayhids) can be commendable, but neither the one nor the other is ever obligatory (as was apparently advocated by al-Murtad̓ā). At the same time, Ṭūsī has constant recourse to reasoned argumentation based on id̲j̲tihād and he begins to sketch the notion of the “general representation” (al-niyāba al-ʿāmma) of the Hidden Imām entrusted to jurist-theologians who may, if the need arises, exercise the prerogatives traditionally reserved for the historical Imāms. In completing and modifying the work of al-Mufīd and of al-Murtad̓ā, Ṭūsī succeeded in endowing Imāmī law with a structure and a scope of activity practically independent of the figure of the Imām. Thus his work was to provide rationalist Imāmism, known from the following century onward as al-uṣūliyya, with solid intellectual bases, enabling it to experience a lengthy evolution which would lead ultimately to an ever-increasing assumption of power by Imāmī mud̲j̲tahids in the economic, social and political fields. The immense and lasting influence of the work of Ṭūsī earned him the honorific nickname of S̲h̲ayk̲h̲ al-Ṭāʾifa [al-Imāmiyya] or simply al-S̲h̲ayk̲h̲ .