logic: identities in syllogism

<<ideas exist,
god is an idea,
therefore, god exists

Lizards exist.
The Lochness Monster is a lizard.
The Lochness Monster exists

Ideas of lizards exist
The Lochness Monster is an ideation of a lizard
Therefore, the Lochness Monster exists. [As an idea of a lizard not as an actual lizard!]>>

This proposition on page one of “Therefore God exists…” (in topic Religion) has been bothering me.

It seems to me that you cannot predicate existence to a universal (except God) like “ideas exist” or lizards exist", because existence is not part of the nature.

Has this been bugging anybody else?


I think that is what predicate logic is for.

Existence is not part of nature?? Maybe… but it is assumed as pertaining to the domain of discourse (the universe). Don’t listen to me though, I got a damn D in the class (argg my GPA weepeth)

cs.odu.edu/~toida/nerzic/con … logic.html

the website states, in part:
<<For example, if P represents “Not all birds fly” and Q represents “Some integers are not even”, then there is no mechanism in propositional logic to find out tha P is equivalent to Q>>

Why would that website and predicate logic want to show that two unrelated premises are equivalent?

It also mentions predicate logic is used for math and comp sci. Is that really proper to use in philosophy? Isn’t it just a kind of shorthand for mechanic operations?

I’m still not quite sure what predicate logic is vis-a-vis prepopsitional logic. You realise, of course, I’m speaking of classical (“Aristotelian”) logic. I fear you might be doing set-theory.

Also, I hope this was just a typo in your response. Not “Existence is not part of nature??” But existence is not part of THE nature (definition) of an idea or a lizard.

Thanks for the response, Gate Control, you always seem to have something interesting to say.


Your problem is the first statement which is left a bit ambigious.

It could be:
All ideas exist.
(Which is patently fasle)

Or it could be:
Some ideas exists
(Which makes the syllogism invalid)

Some frogs are blue
Sally is a frog
therefore, Sally is blue. bzzz wrong


Ahh here we go,

The whole problem of existence ( for some, and all) is handled in the symbolization

It can also be shown using the basic logical symbols employed in Aristolean logic, but the symobolization is chained together and can be (depending upon the semantics or vice versa) infinite. Thus the absolute importance of realizing what the domain of discourse is. (“For evey X…” or “There exists an X…”) Basically, existence is a property attributed to an object. You can formalize the relations between objects as well.

Over the course of two days my prof tore down Syllogistic logic and rebuilt it with just this symbol /
and built it back up based on nothing but that for a symbol (well he also had to change a few formation rules to fit everything neatly) .

There are some transformation rules between the existentials and universals which allow for proofs to be made following certain rules.

I suppose this probably doesn’t explain much, hell I took a semester of it and ultimately didn’t grasp it all as well as I should have. But what I do recall quite well is that predicate logic (besides its use in math and Comp sci) serves to symbolize and thereby validate or invalidate properties (such as existence) ascribed to any sort of object. Or all objects if within the domain of discourse.