The Principle of Existential Inertia

The principle whatever is actual has potential to be actual is only true of things that are a mixture of potential and actual. In the example Alex cites, it's true that nothing about the chair's color changes when it's potential to stay yellow is actualized. But the ongoing yellow state of affairs still represents a reduction of potential to actual that requires something else to actualize it. This becomes more evident when we look at the property of the chair's existence. Something else must actualize the chair's potential to exist, not just at this moment, but at any moment into the future. God being purely actual is the only explanation for the existence of every other actualizer.