Preamble note to the First Way

1.) Something changes when it becomes something else.

2.) But the something and the it give rise to a puzzle. Take boiling water becoming ice. Is the water what changes to something else? No. As water, one has just what he had before. So is the boiling what freezes? That’s even more nonsense. Boiling things can’t freeze.

3.) This generalizes: an actual subject of change is what perseveres throughout the change and so is not different, and the property it has cannot be the contrary it changes into. So the reality of change involves the reality of something non-actual.

4.) So we get what is in motion is potential, not merely in the sense that it’s a necessary truth, but in the stricter sense that the potential suffices to explain the subject of change.

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