In this paper, I set out to explicate what I take to be a distinctive argument that Avicenna offers for the existence of nature (ṭabī ͑a) as a causal power (quwwa) in bodies (ajsām). In doing this, I first clarify the philosophical and historical context of the argument, showing that its two main targets were the Aristotelian tradition on the hand and the Ash ͑arite theological (kalām) tradition on the other. With regards to the Aristotelian tradition, which took the existence of nature as a given, I show that the shaykh departs from it in this regard for at least two reasons. The first has to do with a certain feature of how Avicenna conceptualized, consistent with the Aristotelian tradition, the hierarchical relation between the various sciences. That feature is: principles of a lower science must sometimes be proven by a higher one on the hierarchy. The second reason has to do with the influence of Avicenna’s theological contemporaries; for although such thinkers held, with the Aristotelians, that bodies display various kinds of activity or motion, they were anti-realists about any sort of powers and so of nature understood as a power. That is, they denied, against the Aristotelians, that bodies behaved in their characteristic ways in virtue of some internal power identifiable with ‘nature’ in the technical Aristotelian sense. Instead, bodies, they argued, do what they appear to be doing in virtue of a single, powerful, and transcendent being, i.e., God. Avicenna had to meet this challenge, and I show that he meets it in a unique manner - namely, by allowing, with the Ash ͑aris, the causal involvement of a transcendent being in the production some effect e from some body x, and yet still showing, against them, that e must occur in virtue of some property F in x, where F makes a real causal contribution to e’s occurrence. In this way, Avicenna attempts to establish the existence of nature qua power and thereby refute occasionalism.