Aristotle notoriously begins his examination of being in the sense ofdunamisandenergeiainMetaphysicsTheta with what he describes as the sense that is ‘most dominant’ but not useful for his present aim. He proceeds to define the not-useful sense ofdunamisas “the principle of change in something else or in itself qua other”, along with other senses derived from this primary sense. But what then is the useful sense? All that Aristotle tells us at the outset is that it is a sense that extends (...) “beyond things spoken of only in relation to motion” and nowhere in Book Theta does he explicitly identify the useful sense as such. This has allowed for very different interpretations of the useful sense in the literature, the primary ones being that it is (1) ‘possibility’, (2) the potential to receive a form, (3) being-potentially-x understood modally as encompassing all specific senses ofdunamis, and (4) capacity for substantial change. The present paper argues that there are significant problems with all of these suggestions and defends an identification of the useful sense ofdunamiswith the sense that Aristotle explicitly opposes to the not-useful sense at the start of Theta 8:phusis(‘nature’). This goes hand-in-hand with an identification of the useful sense ofenergeiawith ‘activity’ as distinguished from motion/change at the end of Theta 6. What makes these senses ofdunamisandenergeiathe useful ones for Aristotle’s present aim is that they are required to explain fully the priority ofenergeiatodunamisin substance defended in Theta 8, they support the identification in Theta 9 ofenergeiawith the good, and they explain the unity of the only substances that Aristotle recognizes as being genuinely substances: natural living substances. (shrink)