How is it possible for things that do not exist to not be the same? How can one differentiate nonexistent entities? How can I know the difference between ghosts and werewolves if neither exist?
Ultimately, it all comes down to what you mean by "exist". Werewolves and ghosts do indeed exist, as fictional objects. And thus they can be distinguished from each other within the fiction, even though neither one exists in the real world.
If you wish to pursue this, I'd recommend you look into Husserl's notion of "regional ontologies", which attempts to capture the different types of existence from a phenomenological perspective.
I have to disagree with the "it depends on what it means to exist" answer on two points: The original question does not come down to what it means to exist, and werewolves and ghosts do not exist. Existence may not be a simple fact of the matter, nor easy to elucidate property, but it is manifestly true there are no werewolves or ghosts. If there were, by which I mean if being part of some universe of discourse were sufficient to establish existence, then among other things, the original question would be un-askable.
I think that what OP is asking is: is it possible for one thing with no properties to be different than another thing with no properties? The use of "possible" here is left wide open: If OP is asking this question metaphysically, conceptually, physically, inherently, relationally, etc., then there are going to be different approaches to answering it.
We might get some insight on the question if we keep Leibniz's Identity Laws (the Identity of Indiscernibles and the Indiscernibility of Identicals) in mind. If werewolves and ghosts have all their intrinsic properties in common, they are the same thing, and being that they both have 0 properties, they have all their properties in common. However, being that they also have none of their properties in common, because all of their properties are a sum of zero properties, and having no properties in common means having a sum of zero properties in common, I'd have to say they belong to the empty set.
A way to possibly undermine my conclusion above would be to argue that they are different insofar as I have been able to count two instances of them, so they must have some properties that they don't share. If you're inclined, I recommend looking to Georges Rey's work on intentional inexistence (2003, 2005, 2008) and Sainsbury (2008) on "reference without referents".
I tend to agree with Michael on this one. An analogy might be helpful.
Take numbers. Does the number 1 exist? Can you find and show me 1? How do you know 1 is different from 0? Different from 2? I've never encountered a 1 or a 0 in real life. I've seen a representation of it, perhaps, but never the thing itself.
We take these things to be different from each other by virtue of their properties. For example,
1 > 0
But how can we prove it? The law of trichotomy is that for two entities a and b , a is either less than b, greater than b, or equal to b. This law does not hold for all sets of numbers. For the sets of real numbers, integers and rational numbers it is taken as an axiom. It cannot be proven unless under certain conditions without falling into circular reasoning.
So how does a mathematician differentiate these nonexistent entities? The example I use here are a certain group of numbers and the operations that can be performed upon them. Michael has called this a "context" and in math we call them groups, fields, and rings. Such differentiation is done very carefully. A mathematician will commonly attempt to prove (1) existence and (2) uniqueness for this very reason. Both are necessary to make distinctions between these abstract entities.
Now that we might see a way to differentiate between entities that don't exist in the real world but can still be thought of, the question for me is are there entities that neither exist nor can be thought of? And how would we ever know?
Question 1. "How's it possible for things that don't exist to not be the same?"
Simple. Upon reading a fictional novel, can you not make a separation of the figures within? And since you cannot prove, beyond a shadow of doubt, the existence of those human constructs also means that all of them can be put in the same classification of fictional characters.
Question 2. "How can one differentiate nonexistent entities? "
See my statement above.
Question 3. "How can I know the difference between ghosts and werewolves if neither exist?" Can you not notice a clear separation between the actual objects of reality, and those from a fictional narrative? Because if you can't ... then you're truly lost. And I'd seek help if I were you. Good day
Obviously you can't differentiate things that do not exist, given that there is nothing to differentiate. However you can differentiate descriptions.
(1) "Ectoplasm" was supposed to be a substance or spiritual energy "exteriorized" by mediums.
(2) "Phlogiston" was supposed to be a fire-like element called phlogiston, contained within combustible bodies and released during combustion, which has negative weight.
Clearly if there were such things as ectoplasm and phlogiston, we could differentiate them. There aren't, but we can still differentiate their descriptions.
Reasoning/judging rely on language rather than on physical existence of objects (something you can see/touch/smell etc). That is why you can talk about ghosts, elves or even microbes that maybe you have never seen. Same thing about simple expressions as "hi" and "bye". All of theses expressions are elements in your communication system, signifiers that correspond to concepts and have different values. Actually this linguistic organization is determinant of the delimitation of objects of thought, so that in some culture there may be no difference between green and blue and both are considered the same color. I guess there's a problem with the lack of definition of existence in your question.
I think this goes back as far as Russell and his theory of definite descriptions (“On Denoting” 1905) which in part tried to deal with non-existent entities and to preserve the law of the excluded middle (vis. statements about non-existent entities seem to be neither true of false). If you have a concept to which nothing existent corresponds then that concepts extension is said to be the null set. So the extension of the concepts ghost and werewolves are equivalent- they are both “non-differentiable” members of the same null set.
In set theory, we can define our sets with any arbitrary characteristics. Any entity that has those characteristics is, indeed, in our set. If I define a werewolf as:
- Large teeth
- Human by day
- Wolf by night
- Changes during a full moon
I would not be unjustified in saying that werewolves, by definition, have all of these characteristics. That's not to say that there are any entities in reality that exist with these characteristics, but nonetheless werewolves are still defines by these tenets. Also, I'm certainly not a Platonist, so I don't believe that werewolves exist in another 'realm' simply because they have a concept, but they do, indeed, have defined characteristics, despite not existing in nature.
All things, not limited to physical items but conceptual models as well, have definite characteristics even if they do not or can not exist within reality. For example, a square with three sides has definite characteristics, namely it is square and has only three sides, yet it is inherently contradictory and cannot manifest in reality. We differentiate them based on these characteristics, and existence becomes irrelevant.
I think the fictional entities line of thought is helpful, on a general level, but if you look too closely things start to get confused, and you suddenly have to develop an entire ontology.
Part of the answer that hasn't been mentioned yet in precisely this form, is that it is possible for us to have different propositional attitudes to fictional entities, in the actual world. I believe and know different things about Achilles than I do about Superman.
There are many formulations of ontology, but specifically to categorize "unseen" objects you may find Alexius Meinong usefull:
Briefly, he held the being (having the capacity to be thought of), of an object, was prior to it having existence. He categorized objects into three types of being:
Absistence - Impossible objects like square circles. Having a subset that has:
Subsistence - non-temporal entities such as mathematical objects. Of which an even smaller subset having:
Existence - material and temporal expression, those things you can actually shake a stick at.