Proposition XVIII.

Everything that passes over from a state of potentiality to that of actuality, is caused to do so by some external agent; because if that agent existed in the thing itself, and no obstacle prevented the transition, the thing would never be in a state of potentiality, but always in that of actuality. If, on the other hand, while the thing itself contained that agent, some obstacle existed, and at a certain time that obstacle was removed, the same cause which removed the obstacle would undoubtedly be described as the cause of the transition from potentiality to actuality, [and not the force situated within the body]. Note this.