Existence is the state of having being or reality in contrast to nonexistence and nonbeing. Existence is often contrasted with essence: the essence of an entity are its essential features or qualities, which can be understood even if one does not know whether the entity exists.
One of the topics covered by theories of the nature of existence concerns the ontological status of fictional objects like Pegasus.
Bertrand Russell proposed his theory of descriptions to dissolve paradoxes surrounding negative existential statements.
According to Alexius Meinong, there are some entities that do not exist.
Plato and his student Aristotle disagreed on whether form and matter depend on one another for their existence.
Reality is the sum or aggregate of all that is real or existent within the universe, as opposed to that which is only imaginary, nonexistent or nonactual. The term is also used to refer to the ontological status of things, indicating their existence. In physical terms, reality is the totality of a system, known and unknown.
A brain in a vat that believes it is walking