This chapter explicates a distinctive argument that Avicenna offers for the existence of nature as a causal power in bodies. In doing that, the author shows the argument has two main targets: the Aristotelian tradition on the hand, who thought that the existence of nature, as an intrinsic principle of movement, was self-evident, and the Ash ͑arite occasionalist theological tradition on the other, who were anti-realists about all creaturely efficacious power, locating all efficacy instead in an extrinsic transcendent agent. The (...) argument draws on two key premises: a regularity of events thesis and a version of the principle of sufficient reason. Based on these two premises, Avicenna offers a response to the issue in a way that attempts to preserve something from both traditions. For it allows, with the Ash ͑aris, the causal involvement of a transcendent being in the production of some effect or range of effects from some body; and yet still maintains, against them and with the Aristotelians, that the effect must occur in virtue of some property of that body, where the property in question makes a real causal contribution to the effect’s occurrence. This amounts to a properly Avicennian account. (shrink)