Can "true" nature be circumscribed ? Can we get the ontic fact right ?

But to flip this around If Santa exists then Santa is fat doesn't make sense.

It makes sense, all right. And it is true. If an emaciated Santa claimed to be Santa, we would not believe him. Any Santa would be fat, since that is how Santa is depicted; as a fat jolly old elf. Of course, Santa need not be fat, but unless he were, we would think him an imposter (or maybe that he had had a hard night).

But, of course, what is a necessary truth is that if Santa exists, then Santa has properties. Not necessarily being fat, as I explained, but necessarily as have some properties. For "nothing has no properties" (Descartes).

As for "exist". "Exist" is, of course, grammatically a predicate. Everyone knows that. But that is surface grammar. The question is whether "exist" is a predicate, deep grammar. Kant and Hume both agreed it was not. But it is arguable that although "exist" is not a predicate of objects, as are ordinary predicates, it is a meta-predicate. That is to say, it is a predicate of predicates. And it is a predicate that all predicates that are instantiated have. Thus, for example, to say that God exists is to say that the predicates of omnipotence, omniscience, etc. etc. are instantiated. That seems to me to be a reasonable thing to say. So, is existence a predicate? Yes. But it is not a predicate of objects. It is a predicate of predicates.

ok, what is a property?

my dreams are very vivid, is that a property? and anyway, dreams exist, right? but they arent real...thoughts exist, memories exist...concepts also must exist, like the concept of santa claus or the concept of god. though it doesnt follow that because we have a concept of something it has to exist. we can design a concept like creating a work of art. in fact, now that i think of it we design and create our own sense of self-
not necessarily consciously or deliberately, but it is the mental faculty of the ego if i understand right.

i think existence is not easily definable. except what kenneth said sounds true to me-everything exists, and nothing cant exist. so there isnt any nothing...

What I said was not that everything exists, but that everything that exists, exists. Elves do not exist. Santa does not exist. But then, as I pointed out, elves and Santa are not anything.

"Exist" is definable, for is has been correctly defined, although how easy that was, is another question. Clearly, people have not found it easy to define, "existence" since many have tried, and failed. But, we now know that to exist means, "to have properties". So, whether or not it was easy, we have succeeded in doing it. As Spinoza pointed out, "All excellent things are as difficult as they are rare, for unless they were rare, everyone could accomplish them". And, by rare, Spinoza did not mean, "underdone".

so the concept of santa exists and has properties-but santa does not exist and also has no properties. isnt that the same as assuming that because santa does not exist, therefore he does not have properties? otherwise, why would santa not have the same properties as the concept of santa? or the opposite properties? suppose everyone has the concept of santa being fat, but in reality he is thin and avoiding the papparazzi?

maybe i am confusing a concept with a conception?

Senses can be deceptive for example situation like hallucination or dream. I don't think seeing yourself is solid proof of existence.

I am not assuming that because Santa does not exist, he does not have properties. I am asserting it. You asked for a definition of "existence" and I gave you one. Namely that to exist is to have properties. What is your objection to that definition. Suppose I was asked, what is the definition of "son" and I said, "male child". Would you reply by saying, you are just assuming that sons are male children? Reason that the concept of Santa does not have the same properties as would Santa, if there were such a person, is the very same reason that the concept of an elephant and elephants do not have the same properties. Namely, that the concept of an elephant, and an elephant are very different kind of things. I thought you had agreed that concepts and what the concepts are concept of, are different. After all, you would not want to say that because the concept of Santa exists, that Santa exists, would you? No more than you would want to say that because the concept of The Fountain of Youth exists, that there is a Fountain of Youth.

ok, what is a property?

my dreams are very vivid, is that a property? and anyway, dreams exist, right? but they arent real...thoughts exist, memories exist...concepts also must exist, like the concept of santa claus or the concept of god. though it doesnt follow that because we have a concept of something it has to exist. we can design a concept like creating a work of art. in fact, now that i think of it we design and create our own sense of self-
not necessarily consciously or deliberately, but it is the mental faculty of the ego if i understand right.

i think existence is not easily definable. except what kenneth said sounds true to me-everything exists, and nothing cant exist. so there isnt any nothing...

imo,
A property is a predicate that applies directly to the subject.
For example: In 'a has the predicate B' (Ba), B is the primary predicate and a is the primary subject. But, in ~(Ba), B is not the primary predicate.

A primary predicate o f x is of the form F(x) and a secondary predicate of x is of the form f(F(x))).
Primary predications (properties) entail existence, but, secondary predications do not.
That god is omniscient implies god exists, but, god is not a horse does not imply god exists.

I agree that dreams do exist and they are real, even if their contents are not real or non-existent.

Concepts, names, and descriptions, do exist and their function is to refer or not.

The reference of Santa is the description of the mythical character given the mythical properties that are stated in the myth.
Santa wears a red suit (within the myth), is confirmed true by the myth alone.
Santa wears a red suit, is false in reality...because Santa is not unique.

I agree that "everything exists" is true within predicate logic because all names refer, but if we allow quantification over non-referring terms as well as referring terms then "everything exists" is false.
For example: ((all x)(x exists) -> Vulcan exists) & ~(Vulcan exists), therefore, (everything exists) is false.

In predicate logic, EyEx(x=y) or Ey(y exists), ie. something exists ..is tautologous.
It is not the case that something exists, ie. nothing exists..is a contradiction.

As for "exist". "Exist" is, of course, grammatically a predicate. Everyone knows that. But that is surface grammar. The question is whether "exist" is a predicate, deep grammar. Kant and Hume both agreed it was not. But it is arguable that although "exist" is not a predicate of objects, as are ordinary predicates, it is a meta-predicate. That is to say, it is a predicate of predicates. And it is a predicate that all predicates that are instantiated have. Thus, for example, to say that God exists is to say that the predicates of omnipotence, omniscience, etc. etc. are instantiated. That seems to me to be a reasonable thing to say. So, is existence a predicate? Yes. But it is not a predicate of objects. It is a predicate of predicates.

Are there any other predicates of predicates other than "exist" and its synonymns?

I think "beautiful" may be such a predicate. To say that X is beautiful means (I think) that X has qualities that make it beautiful. But beautiful is what is sometimes called a "supervenient" predicate. It cannot by itself predicate an object, but it is a predicate of an object because the object as other predicates.

It would seem that "good" and possibly "true" would also be supervenient.

X exists = X instantiates some properties. And X does not exist =X instantiates no properties.

Yes. If we define existence as having a property then, Exists(the x:Gx) <-> EF(F(the x:Gx)).
And, ~(Exists(the x:Gx)) <-> ~EF(F(the x:Gx)).

If 'the flying pink elephant' has the property of flying, then it exists.
If 'the flying pink elephant' has the property of being pink, then it exists.
If 'the flying pink elephant' has the property of being an elephant, then it exists.

If 'the flying pink elephant' has any property at all, then it exists.

B(the x: Bx & Cx) -> E!(the x: Bx & Cx), is a theorem.