That which can possibly be need not actually be. In this way, possible being has a certain indifference towards both actual being and non-being. From this, we can see that possible being must necessarily be caused being. Remember that what something is is separate from whether or not it exists. If the definition of something is simply that it exists, we do not search for a cause of such a being, for it does not receive its existence from another but exists in itself. Possible being, since it is not a being such that its definition is that it exists, if it was it could not be possible being, must be caused by necessary being. From this, we see that necessary being is prior to possible being.
All material being is possible being since matter is, by nature, in potency towards some form or act. This composition of matter and form, potency and act, is just what material being is. All material being as such is being caused.