What does Kant mean by "existence is not a predicate"? 1. gives a quality to god, assuming god exists at the same time, since it can't have a quality without existence.
2. presupposes that if god does not exist, then its quality does not exist either.
2.1. There is a hidden assumption or premise: that the quality necessarily exists (i.e. that something is the greatest thing imaginable) as that is a concept without a doubt that anyone can entertain the thought of. You can rank all things for greatness; and if greatness is not equal between things, then there must be one that has the greatest amount of greatness.
3. It is impossible for god not to exist.
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The "predicate" is a part of lines in a syllogism. Syllogisms consists of three lines, and they have strict rules of structures.
All Swedes are protestants.
Some Protestants are blonde people.
Therefore some Swedes are blonde people.
This is an invalid inference, but one of the forms of syllogisms. And syllogisms can be valid or invalid in their conclusions.
in the first line, "Swedes" is the subject, and "protestants" is the predicate.
In the second line, "protestants" is the subject, and "blonde people" is the predicate.
In the third line "Swedes" is the subject, and "blonde people" is the predicate.
In each line, the subject can be set to "all", "no", or "some". "Some" means "at least one", and it is a bit confusing, because in English "Some people are smart" presupposes more than one persons to be smart; but in syllogisms "some" strictly means "at least one".
The predicate, however, must be without a qualifier such as "some", "all" or "no".
Furthermore, in classic syllogisms, both the subjects and the predicates MUST be plural nouns. Countable plural nouns.
Thus, "exists" is not a predicate, in syllogistic terms, because it is not a noun in plural form.
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Since Aristotle, who invented syllogisms, for thousands of years, this was the shape of logic. Aside from the law of non-contradiction and of the excluded middle, the only form of acceptable arguments had been syllogisms.
This is not the case any more.
But I don't know if in Kant's time this was true or not, or if it was Kant that he himself destroyed the "hegemony" of syllogisms as a valid argument in logic.
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To show what I mean:
"Socrates is a man.
All men are mortal.
Therefore Socrates is mortal."
is an invalid syllogism, although logically it is true. In a proper syllogistic form, it should read:
"All men (or beings) like Socrates are men.
All men are mortal beings.
Therefore beings like Socrates are mortal beings."
Likewise, we can transform Kant's Ontological argument into a syllogism:
All things that are the greatest things imaginable, are god-things.
(The second sentence can't be transformed)
Therefore god exists.
Here, we encounter two problems: 1. the argument is NOT in a syllogistic form, yet 2. it is a reasonable and acceptable argument. This could have presented a problem for Kant or his contemporaries; and since this argument exists, I assume that the invention of this ontological argument was the in-road into the new age of logical arguments where syllogistic forms were no longer necessary in an argument.