  Natures as inner principles of change and rest are contrasted with active powers or potentialities (dunameis), which are external principles of change and being at rest (Metaphysics 9.8, 1049b5–10), operative on the corresponding internal passive capacities or potentialities (dunameis again, Metaphysics 9.1, 1046a11–13). When a change, or a state of rest, is not natural, both the active and the passive potentiality need to be specified. Natures, then, in a way do double duty: once a nature is operative, neither a further active, nor a further passive capacity needs to be invoked. Even so, as will be clear from Aristotle’s discussion, this general thesis  will require a host of qualifications.