# Why is it true that anything that changes must be divisible according to Aristotle?

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?

My expertise lies more towards the ethics side of things (especially with Aristotle), but I think Aristotle's point here generally makes sense so I will see if I can spell it out better. One confusing and important point is that "change" in English is broader in meaning than what Aristotle means, which could be called "alteration" instead.

On Aristotle's account, change has two dimensions:

1. a thing that continues to exist (it is there at points in time A and B).
2. an alteration such that something about this thing is different at points in time A and B.

If you don't have 1, then you're looking at the destruction of one thing and the generation of something new rather than a change.

If you don't have 2, then there's been alteration in the thing between point in time A and B, thus no change.

Aristotle articulates this conjunction as saying the thing "must be divisible." What this means is that, there's got to be some sort of continuous thing, but that it must have aspects that can change while it's the same thing.

An example or two might help.

A red car undergoes change when you paint it blue. The car still remains (reaching condition 1), but the car's color has changed (reaching condition 2). The color of the car is thus one divisible aspect of the car.

A car does not undergo change when it is incinerated (imagining that it is completely turned into its elements at 50000 K). Once burned, there's no car (breaking condition 1). Thus, there's no aspect that changes -- the thing ceases to exist. And nothing divisible has happened. (Aristotle calls this "destruction" -- not "alteration")

A car also does not undergo change when it is made in the factory. It becomes a car and wasn't one before (= "generation" or "creation" for Aristotle).

I think you're losing Aristotle's track when you suggest "two items" but maybe I am misreading you.

(n.b., I'm not necessarily endorsing this as a true and complete model of change).