Aristotle states in Physics 6:4:
Further, everything that changes must be divisible. For since every change is from something to something, and when a thing is at the goal of its change it is no longer changing, and when both it itself and all its parts are at the starting-point of its change it is not changing (for that which is in whole and in part in an unvarying condition is not in a state of change); it follows, therefore, that part of that which is changing must be at the starting-point and part at the goal: for as a whole it cannot be in both or in neither. (Here by 'goal of change' I mean that which comes first in the process of change: e.g. in a process of change from white the goal in question will be grey, not black: for it is not necessary that that that which is changing should be at either of the extremes.) It is evident, therefore, that everything that changes must be divisible.
I'm having trouble understanding his argument. I understand him to be saying that change has two states, before and after the change ("starting point" and "part at the goal"). Since these two states can't coexist in one item (since one item can't be both pre and post change), there must be at least two items in any changing object, one which is at the starting point and one which is at the ending point. Therefore anything which changes must be divisible.
Why must these two states coexist at all? It seems reasonable to say that an object transitions from one state to the other. Therefore if we were to have an indivisible object, it would transition, as a whole, from pre-change to post-change. What am I missing here?