Everything real and nothing unreal belongs to the domain of Being. But there is little useful that can be said about everything that is real, especially from within the philosopher's study, so it is not apparent that there can be such a subject as Being by itself. Nevertheless the concept has a central place in philosophy from Parmenides to Heidegger. The central question of ‘why is there something and not nothing?’ prompts logical reflection on what it is for a universal to have an instance, and a long history of attempts to explain contingent existence by reference to a necessary ground. In the tradition since Plato this ground becomes a selfsufficient, perfect, unchanging, and eternal something, identified with the Good or God, but whose relation with the everyday world remains obscure (see ontological argument, cosmological argument, principle of plenitude). Modern logic gives little comfort to these speculations, and prompts suspicion that the question of why there is something and not nothing is either ill-formed or profitless, since any intelligible answer will merely invite the same question. A central mistake in the area is to treat Being as a noun that identifies a particularly deep subject-matter. This is parallel to treating Nothing as a name of a particular thing, perhaps an object of dread or fear. The modern logical treatment of these notions by means of quantifiers and variables provides a defence against this error and others. The less abstract part of the study of being concerns the kinds of things whose existence we have to acknowledge: abstract entities, possibilities, numbers, and so on, and disputes over their reality form the subject of ontology.