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According to SEP

There are two sets of reasons for denying that existence is a property of individuals. The first is Hume and Kant's puzzlement over what existence would add to an object. What is the difference between a red apple and a red existing apple? To be red (or even to be an apple) it must already exist, as only existing things instantiate properties.

Contra this, one can try out the sentences 'White unicorn' and a 'white existing unicorn'.

Here the adjective, or qualifier 'existing' actually adds something to the sense of the sentence; after all, for those who know that unicorns don't exist, to be told that they are adds something to their knowledge of the world.

To put the second sentence into subject-predicate form; one should say 'A white unicorn exists'...

(To which, the correct response, considering that I am a reliable informant, should run along the lines like 'My God - unicorns! really...where? Did you get some video footage...')

Notably the first sentence 'white unicorn' is just a sentence fragment, or a description; which might indicate some connection with Russell's theory of descriptions.

Why then isn't existence a predicate?

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    You can see Francesco Berto, Existence as a Real Property : The Ontology of Meinongianism (2013) for a recent discussion of the view that existence is a "real property of objects that some things possess, and others lack.". – Mauro ALLEGRANZA Mar 8 '15 at 21:53
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    "Only existing things instantiate properties." How true that is. Yet, the predicate "exists" does have meaning when it comes to such expressions as do "unicorns, ghosts, fairies, goblins, Santa Claus, Yeti, Big Foot, the Loch Ness Monster, the great pumpkin, etc. not exist. Personally, I find great satisfaction in knowing ghosts do not exist. – Michael Lee Mar 8 '15 at 22:30
  • Just musing, I wonder if their answer would be that a thing that does not exist (unicorn) is axiomatically rejected from their universe, implying we have to fall back on things like "the imagined form of a unicorn." – Cort Ammon Mar 9 '15 at 1:29
  • @ammon: there are such things as fictional universes – Mozibur Ullah Mar 9 '15 at 14:16
  • @lee: see above comment; I feel the same way about demons, ogres and hobgoblins; but not so much about griffins and phoenixes. – Mozibur Ullah Mar 9 '15 at 14:19
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I think the issue mostly arises from having an insufficiently rich language to describe just-the-actual-world vs. model-worlds. When you can't cleanly distinguish between the two, existence becomes problematic. If there is no red apple, simply saying "a red apple" is already a problem since it does not refer to anything.

One solution is to use modal logic. One can define a non-problematic (or at least less-problematic) type of existence like so:

E(t) := ∃x(x=t)

But one needn't necessarily go to such lengths: just say that existence is a property of your model objects (regarding how they correspond with reality) and don't make such a claim about actual objects, and you're pretty much free of problems.

  • Question about how this works. Is there a difference between "Captain Ahab is obsessed with a white whale," and "Captain Ahab had Wheaties for breakfast?" One is a true statement about a fictional entity. The other is an unknowable and meaningless statement about a fictional entity. And how about "Captain Ahab is the cabin boy on the Pequod? That's a false statement about a fictional entity. How does modal logic distinguish among these? – user4894 Mar 9 '15 at 3:08
  • @user4894 - I don't really like modal logic as a solution to the richness problem, and they don't entirely agree on how to treat fictional entities. See plato.stanford.edu/entries/fiction for way more. Personally, I think that there are no good and reasonbly-widely-accepted formalisms yet, so we do just as well to accept some level of informality as long as we get the basic character of the reasoning right (i.e. "Captain Ahab is obsessed with a white whale" is true in some sense, while "Captain Ahab had a large mole behind his left knee" is undetermined.) – Rex Kerr Mar 9 '15 at 7:50
  • Isn't it because "a white unicorn" is a concept, an imagining, and "An existing white unicorn" is an actual item, a thing in reality ? Ie they're different things, not interchangeable in thought or speech (but they share facets which is why an existing white unicorn might be called a whie unicorn), so adding that the existing one exists is unnecessary? – user2808054 Mar 9 '15 at 10:09
  • @user2808054 - One might note that our minds are not the same as reality, and thus "an existing white unicorn" is a mental model just as "a white unicorn" is. It is a different model, but I'm not sure it's any more different than "a blue Ford Taurus" is from "a Ford Taurus". It is easy to overlook the context which tells us which subset of our models to use (e.g. actual-real-world, possible-real-world, desired-future-world, imaginary-but-realistic-world, etc.). So while the argument you've made has been made elsewhere, I'm not sure it really cuts to the heart of the matter. – Rex Kerr Mar 9 '15 at 10:45
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    @user2808054 - You can often select the model implicitly without worry, but fictional works routinely have direct or indirect claims about the existence of something (the One Ring, Elendil's sword, etc.), so the idea of existence presumably must be model-relative. – Rex Kerr Mar 9 '15 at 19:14
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You say "one can try out the sentences 'White unicorn' and a 'white existing unicorn'. Here the adjective, or qualifier 'existing' actually adds something to the sense of the sentence; after all, for those who know that unicorns don't exist, to be told that they are adds something to their knowledge of the world. "

But the expression 'white existing unicorn' is not a sentence, but rather a noun phrase. As soon as you make it into a sentence, you get existence in either case.

(A) some unicorn is white

(B) some existing unicorn is white.

The first asserts that some, i.e. at least one, unicorn is white. The second asserts the same thing, but adds the redundant information that the unicorn is existing.

If you object that (A) could be true because it says in some story that a unicorn is white, I reply that in this sense (A) is elliptical for "according to some story, some unicorn is white". This does not assert the existence of unicorns.

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    (A) certainly does assert the existence of a unicorn. What you mean is "All unicorns are white." That is vacuously true and does not assert the existence of a unicorn. Whether things are true in stories is a totally different question, as in "Ahab is Captain of the Pequod," which I mentioned in an earlier comment to the other response. – user4894 Mar 9 '15 at 18:21
  • "As soon as you make it into a sentence, you get existence in either case." -- Um, not all sentences start with the word "some". e.g., in, "He referred to white [existing] unicorns", "existing" is not redundant. – Jim Balter Apr 24 '17 at 6:28
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Grammatically, of course, 'exists' is a predicate. 'The Lincoln Memorial exists' involves a predicative use of 'exists'. Whether existence is a property is another matter. And it's because it is widely assumed that existence is not a property, hence the predicate 'exists' has no property corresponding to it, that 'exists' is (while grammatically a predicate) not a 'real' predicate.

The thinking behind this is as follows.

If I say :

X is blue

X is square

X is heavy

it seems not unreasonable to assume that everything that is blue, square, and heavy exists. Hence X exists : how could X have these properties, how could these predicates apply to it, if it did not exist ?

So if I make a second list :

X is blue

X is square

X is heavy

X exists

what have I added ? It is not possible for the first three statements to be true and the fourth ('X exists') false. 'X exists' has added no information. But if existence were a property, it would add extra information. Therefore existence is not a property; and the predicate, 'exists', corresponding to it, is in this sense not a real predicate.

This explains, I think, why 'exists' is standardly taken not to be a predicate. And there plainly is something wrong in saying 'X is blue, square, and heavy - and also exists'. However, when one says 'it seems not unreasonable to assume that everything that is blue, square, and heavy exists', this seems precisely to concede that existence is a property. If it is a property, why shouldn't 'exists' be a perfectly proper, not at all 'unreal', predicate corresponding to it ?

Meinong can, of course, be brought into play but to extend the answer to include Meinong would take us too far into dense and complex territory. Meinong's views need a separate question.

REFERENCES

Kant, Critique of Pure Reason, A598/B626-A600/B628.

C. Heathwood, 'The relevance of Kant's objection to Anselm's ontological argument', Religious Studies, Vol. 47, No. 3 (SEPTEMBER 2011), pp. 345-357.

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Not so much an answer as an annex to @GeoffreyThomas.

The question exposes some considerations to keep in mind when using "exist(ance)" in a formal context. To illuminate this it is helpful to use the ontology of Alexius Meinong:

https://plato.stanford.edu/entries/meinong/#ObjTheOntBeyBeiNonBei

and specifically his types of objects:

https://en.wikipedia.org/wiki/Alexius_Meinong#Types_of_objects

We can restate the three types as follows:

1) Things that can be named. 2) Things that may be conceived. 3) Things we can literally point out to each other.

Considering something like a square circle we see that it can only satisfy the first type. In this case we could never use a quantifier in relation to the object. "Exist" could only be used as a property and then also only as a negated one.

The table in front of me is clearly within the description of all three types. Here it would be appropriate to use a quantifier such as "exist", but also "exist" is a special kind of property, one that lifts the object out of the realm of contemplation into the physical and sensible. Note Meinong attributes "temporal" too

It is objects that satisfy only the first two types that are a little more tricky. For instance mathematical objects, does a number exist? In some philosophies numbers aren't real, thus the property "exist" would not exist. But then again there may be systems where some concepts are "real" and some concepts not. In this case, "exist" as a property would be applicable on a per concept basis. The actual formal system in use is a deciding factor on how "existence" can be applied, but the ontology can limit the availability of properties.

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I think the problem arises from the assumptions inherent in the philosophical subject-predicate form.

The underlying principle seems to be that the predicate should qualify the subject in the sense that it becomes more specific; it should refine it. So

"A ball is in my garden" is refined by "The ball in my garden is blue".

The problem with existence as a qualifier is that it trumps the subject - we cannot say anything meaningful about the subject before we know whether or not it exists. And

X does not exist

could be said of a great number of different kinds of X without altering the reality being described (this is the sense in which I say it trumps, or is more important than the subject). Non-existence is indiscriminate - whatever it is that is said to not exist, the statement still amounts to the same thing - an absence of anything.

I can only suppose that this is the reason that the assumption of the Subject-Predicate construction in Philosophy seems to be that the subject exists. If existence of the subject X is assumed, X exists is redundant and X does not exist is paradoxical.

However natural language does not make this assumption and so sentences such as "Dragons do not exist" and "There is a solution to this problem" have semantic value.

So, to put it another way, Kant is simply pointing out the assumption in the Subject-Predicate construction.

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I agree with Rex Kerr that "the issue mostly arises from having an insufficiently rich language". However, Kant went to lengths to resolve the issue by distinguishing the 'real' from the actual.

For Kant, the 'real' is the possible, and the actual is the real that has been observed; Existence is made in the interaction of observer and the observed.

So there are two major redefinitions from everyday language here: the real and existence.

To appreciate these redefinitions one has to approach things phenomenologically: one's experiential point of view is the start point. Then you can introduce the real. For example, walking down the street, there is a real possibility that there is a car around the corner. In negotiating the world this possibility is treated as a real thing; the world is full of such possibilities, their sum constitutes 'reality', (in Kantian terminology).

In contrast, actuality is that part of the world that has been observed, confirmed or created.

Turning the corner to discover the car does not add to the concept of the car; the car has not been changed by being observed. Hence, existence is not a predicate.

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