Existence can be defined a number of ways, but can simply be said to be things that 'are'. Often is the case where existing things are also defined to have the property of persisting independently regardless of the observer. Although a seemingly simple concept, existence raises many important metaphysical questions. For example: “Is existence a property of objects?”, “What is the relation between existence and essence?”, “Can objects lack existence?”.
Whether or not existence is a property is one of the more important questions pertaining to existence, and a rather substantial group of philosophers reject that it is. These include: Hume, Kant, Russell, and Aristotle. The answer to this question becomes important for many other fields of thought such as arguments for and against the existence of God. For example, the ontological argument for the existence of God goes as follows: God is a being of the utmost perfection and goodness. Existence is a good-making quality; a being that exists is better than one that does not. Therefore God must exist.
Kant argues against such a claim for God's existence by relying on the idea that existence is not a property, but an essential part of a being that cannot be removed from it without destroying the being. Here we can see that the discussion of whether or not existence is a property is pivotal to the truth of the ontological argument for God. Russel, on the other hand, takes a different approach in asserting that existence is a property of concepts rather than individuals. In a sense then, existence is a “second-order” property. This is just one example of a question raised about the nature of being and existence, but the list for this metaphysical concept goes on.