Avicenna is a strong proponent of what some of the later ones call qā ͑idat al-wāḥid or ‘rule of the one’ (RO). The gist of RO states: from the one only one directly proceeds. In the secondary literature, discussion of this Avicennian rule is usually limited to a particular application of it i.e., the issue of emanation. As result, it’s not really clear what RO means, nor why Avicenna endorsed it. In this paper, I try and remedy this situation by doing two things – one on the taṣawwur front, the other on the tasdīq. First, explain just what the terms of RO amount to – that is, its subject and predicate. In doing this, I distinguish between a narrow and a broad understanding of RO, and the show that, on the Avicennian view, the scope of RO is broad; it is meant to be a general principle of efficient causality. This is why it is appealed to in various contexts to establish substantial philosophical theses. Second, I consider an argument Avicenna offers for RO in the Mubāḥathāt. In unpacking it, I uncover some of its realist presuppositions, and then further clarify it in light of a critique first raised by Bahmanyār and then later made famous by Fakhr al-Dīn Rāzī. I then conclude by seeing whether the Avicennian has the resources within the initial premises of the argument to meet the objection that’s raised.