The rule “al-wāḥid lā yaṣdur ʿanhu illā al-wāḥid” (the One does not emanate anything but the One) is one of the most fundamental rules of Islamic philosophy, according to which a single cause does not produce anything other than a single effect. This rule, rooted in Peripatetic philosophy, has always been a site of discussion and dispute among philosophers and mystics throughout the history of Islamic thought. Khomeini, as a mystic-philosopher, has a distinguished and unique approach to this rule. While accepting the principle of the rule, he examines it through two formulations: “al-wāḥid lā yaṣduru minhu illā al-wāḥid” and “al-wāḥid lā yaṣduru illā min al-wāḥid,” and with his mystical perspective, he offers a new reading of it. In this view, the rule of the One is not only not incompatible with the theory of the personal unity of existence, but by interpreting “unfolded existence” (wujūd munbasit) as the first emanation, the multiplicities of the world can be justified without violating the rule. Relying on the mystical foundations of Muḥyī al-Dīn Ibn ʿArabī and his followers, especially Ṣadr al-Dīn Qūnawī, Khomeini introduces unfolded existence or the Muhammadan Reality (ḥaqīqat Muḥammadiyya) as the first determination and manifestation of the Supreme Truth, which is devoid of quiddity and limit and, as the intermediary of effusion, becomes the origin of the manifestation of multiplicities. This research, using a descriptive-analytical method and relying on Khomeini’s written works, especially his philosophical treatises and his commentaries on Miṣbāḥ al-Uns, explains his mystical approach to the rule of the One and shows how, by synthesizing the views of philosophers and mystics, he offers a horizon beyond both, in which the rule of the One can be presented in two different philosophical and mystical stations and ranks, and the confusion of these two stations has been the source of many errors in understanding the rule.