The principle of causality, part two

Another way to understand why nonbeing cannot cause being is by noting that everything that "comes to be" must have a cause. If it comes to be it is not a Necessary Being, which by its nature must always be. So what comes to be is, by definition, a contingent being, a being that is capable of existing or not existing. For every contingent thing that comes to be there must be some efficient action that causes it to pass from a state potentiality (potency) to a state of actuality (act).
For, Aquinas noted, no potency for being can actualize itself. To actualize itself it must be in a state of actuality and before it is actualized it must must be in a state of potentiality. But it cannot be both at the same time (a violation of the principle of noncontradiction). Hence, one cannot deny the principle of causality without violating the principle of noncontradiction.

https://www.youtube.com/watch?v=FrBpSr2JbCo&index=1&list=PLIpO3BUiq2IHKHZVzyzayj5yemo03pxfY