Aristotle introduces Physics I as an inquiry into principles; in this paper I ask where he argues for the position he reaches in I 7. Many hold that his definitive argument is found in the first half of I 7 itself; I argue that this view is mistaken: the considerations raised there do not form the basis of any self-standing argument for Aristotle's doctrine of principles, but rather play a subordinate role in a larger argument begun in earnest in I 5. This larger argument stalls in I 6, which ends in aporia; I argue that the problem lies in the fact that Aristotle's reasoning in I 6 thoroughly undermines his reasoning in I 5 (on which I 6 is ostensibly supposed to build). I further argue that the materials necessary for resolving this problem, and thereby allowing the argument begun in I 5 to reach its proper conclusion, are supplied by the thesis that organizes the first half of I 7. Along the way I offer some remarks about Aristotle's doctrine of principles, arguing that it is about the principles of natural substance (as opposed to coming to be or change). I also offer some remarks about the thesis which organizes the first half of I 7. I argue negatively that it is not anything like a preliminary statement of Aristotle's doctrine of principles. I argue positively that it reflects Aristotle's idea that there are two distinct kinds of effect change has upon things (one constructive, the other destructive). One of these effects lies behind Aristotle's reasoning in I 5, the other comes to the fore in I 6; the achievement of the first half of I 7 is to reconcile these seemingly competing conceptions by finding a place for them both in a unified account of coming to be and its subjects.