There are a variety of senses in which something can fail to exist:
- In fiction: Romeo and Juliet do not really exist, because they are merely characters in a play. So there is fictional nonexistence.
- Suppose one is just daydreaming and one imagines winning the Nobel Prize. This Nobel Prize does not exist: it is just imaginary. So another kind of nonexistence is imaginary nonexistence. (Or one might say, tendentiously but idiomatically, "It has existence only in one's imagination.")
- Consider that old hypothesis, that the Earth is flat. People who thought the Earth was flat thought there was an edge to the world, and if one sailed too far across the ocean one would sail off the edge of the world. The world is not flat but roughly spherical, and the edge of the world does not exist. So that is a another kind of nonexistence: hypothetical nonexistence. In other words, that is the sort of nonexistence that false posits have, when one posits something in a false hypothesis.
- There is a quite different kind of nonexistence, namely, the kind the ancient Greek philosopher Socrates now has. Socrates does not exist now. He did exist, but there is a sense in which he does not exist; namely, he does not exist any longer. So another sort of nonexistence is present nonexistence. Of course--though this presents a separate set of problems (e.g., having to do with free will)--something presently nonexistent might exist in the future. The person who will be President of the United States 200 years from now does not, now, exist. So we might distinguish two types of present nonexistence--the sort had by entities that did exist and the sort had by entities that will exist. Of course, neither of these notions is entirely unproblematic.
One central problem that philosophers face in thinking about nonexistence is generally discussed under the heading fictional truth. How can we account for the truth of the proposition 'Romeo killed himself' when Romeo is nonexistent?