Is all that exists part of the natural world? If there are non-natural entities, what is their difference from natural things? Is the human- independent realm of nature the only paradigm for ontological respectability, as naturalism claims? Can existence be simply explained away by means of formal devices? Philosophers keep struggling with such questions. Still, the two basic notions involved, that of existence and that of nature, have not yet been fully explored. The four essays collected here address the issue from the points of view of the philosophy of mathematics, of analytic ontology, of early modern philosophy, and of contemporary phenomenology. The results will surprise