Sentence:  The master argument of On the Plurality of Worlds is that a <WSD>pluriverse</WSD> composed of infinitely many concrete universes constitutes a “paradise for philosophers” wherein “we find the wherewithal to reduce the diversity of notions we must accept as primitive, and thereby to improve the unity and economy of the theory that is our professional concern - total theory, the whole of what we take to be true." 
Meaning of the word marked with <WSD> tags:  A set of all possible universes.