In philosophy the field of ontology considers what things exist, and what existence implies.[1] Quine has called the question ‘What is there?’ "the ontological question".[2]
For some philosophers, called 'deflationist', this question is linguistic, that is, it concerns the usage of expressions like 'At least one such-and-such exists'.[3] Such phrases are called quantifier expressions.[4]
For other philosophers, ontology concerns the actual existence of real things in the universe. The subject is complicated by discussion of issues like whether compound objects really exist, a question of mereology. For example, does a 'book' exist or only the 'pages' of the book. How do we draw the line between 'reasonable' compound objects and silly ones like 'my nose and the Eiffel tower'?[5] Another confusion called 'Plato's beard' considers whether a statement like 'Pegasus is a flying horse' implies a belief in the existence of Pegasus. In general terms, is it true that statements, regardless of whether they are true, are about something?[6] Another large arena for discussion is the existence of properties and other universals that appear to be instantiated in multiple objects, rather than a particular one,[7] and their distinction from instances (tropes).[8]