 One compelling argument for existence monism (2000: §2.4, also Schaffer (2007)) is that the complete causal story of the world can be told in terms of the world and the laws without appeal or reference to parts of the world. So, if there were parts of the world, they would be redundant and/or  epiphenomenal (see entry).  But we shouldn’t posit explanatorily redundant or epiphenomenal things. So, we shouldn’t posit parts of the world. So, only one object—the world—exists.