Succession is understood through parts (not divided parts, just exteriority) and it is the nature of succession that one part cease and another arise. Though the parts of the extended do not have the same ceasing, the one must still be a negation of the other, and the duration - that is, the existence – of the extended is certainly characterized by such continual ceasing of its parts. The existence of extended being is understood to involve non-being essentially- not because pure nothingness enters into its being, but because a.) its existence (as duration) involves continual ceasing to be and b.) its very extended or physical existence requires one part not being the other, and no part of the extension or the extension itself being identical to the very being of the extended.
While non-being does not enter into the composition of the extended or physical, the physical itself is essentially changeable and so requires some internal, essential part that is undetermined. So we divide the undetermined from sheer non-being, as we do in experience.
Though the physical is self-evident to us – and we are incapable of having a thought without some unbreakable relation to the physical, we can nevertheless know it as it is, and therefore know it as impossible to separate from a train of non-being, and as requiring within its essence something that can only be called a being like a grape can be called wine. In what is higher, beings require this in the order of their activity; in the lower they require it not only in their activity, but in their very essence.