Sentence:  Thus, unless there exists some being that exists <WSD>"per se</WSD>", the origination of "esse" in a chain of composites itself remains unexplained and quite mysterious. And the existence of a being that exists <WSD>"per se</WSD>" is affirmed through a denial of an infinite regress of essence-"esse" composites causing other such composites. 
Meaning of the word marked with <WSD> tags:  Being a thing that posits itself and is a principle of its own determination.