In light of the notions of ‘form’ and ‘essence’ as per the existential framework, this paper demonstrates that the proscription of trading with impure substances, and the strict protocols on maintaining a state of purity and being cleansed of impurities, was contextual to the way of life of the seventh century Arabian community and their environment. To begin with, it provides a brief overview of the elements of the existential framework relevant to this paper. Then it presents an etymological and conceptual analysis of the notions of the Arabic words najis (impure) and najāsāt (impure substances), their meanings in the Qurʾān and ḥadīth literature, and the general context of pre-Islamic Arabia in terms of the prevalent attitude, norms and customs regarding cleanliness and hygiene. The next section of the paper focusses on one particular ‘impure substance’ (najāsa) in detail: the impurity of non-Muslims in Shīʿī jurisprudence. It outlines the evolution of the purity status of the non-Muslim, followed by an analysis of the relevant religious texts, jurisprudential arguments and justifications, and it closes with the perspective of the existential framework. Finally, the hermeneutic of ‘form’ and ‘essence’ is applied to the texts of the Qurʾān and ḥadīth literature utilised by legists as Sharia evidences to designate other substances as “impure” in a brief but critical survey.