Brentano’s refutation of the Ontological Argument

Start with the argument in simplest form

TTWNGCBT is something that must exist in reality and not just the mind


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.


  1. dyingst said,

    October 23, 2015 at 7:20 am

    It’s not clear to me that “is” can be replaced with “means” in example A. If it were “a shoemaker” referring to the word then perhaps, but if we’re talking about actual shoemakers, isn’t there more being expressed by “is” than “means”?

    • October 23, 2015 at 9:48 am

      We can re-wrire Brentano’s argument without the “means” language. We’ve always known that one premise in the OA has to assert real existence (so that it can show up in the conclusion) and the other can’t (so that the proof can avoid circularity) but B. calls this “equivocation”.

      • thenyssan said,

        October 23, 2015 at 10:10 am

        Is that re-write or re-wire? Amazing neologism. I’m stealing it.

%d bloggers like this: