While expounding the contents of Aristotle’s Physics, Avicenna endeavours to follow the model of scientific exposition proposed by Aristotle in his Posterior Analytics. Thus he carefully characterises the object of physics, its principles and its domain of investigation. He also dismisses the dialectical method used by Aristotle in the discussion of the principles and refers instead to the metaphysical discipline where these principles are supposed to be demonstrated. Resorting to the Demonstration and the Metaphysics of the Shifa’, I will try to display the epistemological project lying behind the changes introduced by Avicenna in the structure of Aristotle’s Physics.