Start with the argument in simplest form
TTWNGCBT is something that must exist in reality and not just the mind
God is TTWNGCBT
So God must exist in reality and not just the mind.
Brentano claims the argument commits the fallacy of equivocation. Normally this would occur in the middle term, but Brentano sees it as occurring in the word “is”. There are two senses if the word, call them a and b:
a.) In a sentence like “A shoemaker is a person who makes shoes” or “a cheetah is a large cat” the sentence can be re-written by replacing “is” with “means”.
b.) In a sentence like “the car is on the freeway” cannot be re-written in the same way, but involves a claim beyond what means about what is real.
Now either the premises are both a, both b, or we have one of each. If both are a, then the conclusion only explains what the word means, and is not a proof that God exists; if both are b, then the minor premise begs the question. But if we have one of each, then the argument involves an equivocation on “is”. Since Brentano takes it as self-evident that the first premise is a (b) and the second is an (a), he takes it as an obvious equivocation.
Brentano explains further that if one is allowed to mix a and b senses of “is” he can prove anything, e.g.
a shoemaker is a person who makes shoes
But you can’t make shoes if you don’t exist, can you?
Therefore a shoemaker exists.
A unicorn is a horse with a single horn
But in order to have any horn at all you need to exist
Therefore a unicorn exists.
One response to Brentano is that he seems to prove too much: Proving the existence of anything, would seem to require both senses of “is”, e.g.
A black swan is (means) something that looks like a swan, but is black
Oh look, there is something that looks like a swan, but is black.