The Argument from Mutual Obstruction (dalīl al-tamānuʿ) is a classical kalām proof for the divine unicity (tawḥīd) rather than for divine existence as such, though it functions within the broader cosmological project as a complement to dalīl al-ḥudūth. The argument reasons that if there were more than one omnipotent deity, they could in principle will contradictory states of affairs — one willing motion of a body while the other wills its rest. Such a situation would be incoherent, since neither could be omnipotent if the other could obstruct his will. From the impossibility of mutual obstruction, the argument concludes to a single, sovereign divine will.
The argument has Qurʾanic resonance in verse 21:22 — law kāna fīhimā ālihatun illā Allāh la-fasadatā ("if there were therein gods besides God, both would have been corrupted"). It was systematized by early mutakallimūn including al-Jubbāʾī, al-Ashʿarī, al-Bāqillānī, and al-Juwaynī, and later refined by al-Ghazālī. The argument operates within the kalām framework of divine attributes and will, where omnipotence is understood as effective volition. Variants of the argument distinguish between the impossibility of two wills disagreeing (ikhtilāf al-irādatayn) and the impossibility of two wills agreeing without redundancy, generating different inferential moves toward the conclusion of unicity.
The argument has attracted criticism within the Islamic tradition itself. Ibn Taymiyya questioned the modal force of the argument, arguing that the mere logical possibility of conflicting wills does not establish the actual impossibility of multiple deities — perhaps multiple deities would simply will harmoniously by necessity. The Muʿtazila and Ashʿarīs debated the precise modal structure of the argument and its relation to other proofs of unicity. Contemporary scholarship by Richard Frank, Sabine Schmidtke, and others has examined the argument's logical structure and its dependence on specific accounts of will and omnipotence developed within early Islamic theology. The argument is less prominent in contemporary analytic philosophy of religion than its cosmological siblings, though questions about the coherence of polytheism touch on similar issues.
Among other formulations in the cosmological family, the dalīl al-tamānuʿ is distinctive in arguing for divine unicity rather than mere existence. It complements dalīl al-ḥudūth in the classical kalām synthesis: the former establishes that there is a creator, the latter that the creator is one. It differs from the First Cause, Thomistic, Leibnizian, and contingency arguments, which all concern existence rather than unicity. Ibn Sīnā's modal proofs touch on unicity through the analysis of necessary existence, but operate by a different inferential route. In the broader project of natural theology, dalīl al-tamānuʿ represents a specifically theological refinement absent from most Western cosmological arguments.