Avicenna offers a novel definition of “nature” as a power in his Physics of the Healing. Some have seen in this redefinition a radical departure from Aristotle. James Weisheipl, for one, rejected Avicenna’s definition as a mistaken interpretation of Aristotle and as a position incompatible with Thomas Aquinas. In Weisheipl’s view, Avicenna reifies form into a kind of motor of the natural being, a conception earlier rejected by Thomas Aquinas in several works. In this study, I offer a Thomistic defense of Avicenna by investigating the definition of nature and its relation to matter and form.