Here’s an interesting little philosophical puzzle.

If I say, “unicorns exist,” am I actually saying that unicorns exist?

Well, yes, one might say. Obviously, that’s the meaning of it.

When I say, instead, “not everything is not a unicorn,” do I say that “unicorns exist” in the same way? Think about it: There are unicorns. So far, so good. And then, there are all these things that are not unicorns: traffic lights, lions, accountants, philosophy books, numbers. If everything was not a unicorn, then there wouldn’t be any unicorns. If not everything was a non-unicorn, then there surely must be at least one unicorn; otherwise, everything would be a non-unicorn. Still with me?

The two statements are actually logically equivalent, as one can verify by thinking about the non-non-unicorns a little longer. Or, to give a vegan example: If not everything is not an apple, then there must be at least one apple.

If I say, “unicorns exist,” am I actually saying that unicorns exist? 

I can do this also by adding a predicate to the sentence, some feature that unicorns have, for example, to like hackers. So I would say, “there are unicorns that like hackers.” This is, as we now know, equivalent to: “not all unicorns dislike hackers.” But it seems that there is a subtle difference between the two sentences. If someone you just met on the street told you: “You know, not all unicorns dislike hackers,” you’d probably shake your head at the kinds of things people spend their time on, but you wouldn’t call the police. If the same person said: “There are unicorns that like hackers,” that would be a reason to really worry about them. Somehow the first form doesn’t actually commit the speaker to the idea that there really are any unicorns around. “Not all unicorns dislike hackers,” could just be a statement about the unicorn in poetry and art. While “there are unicorns that like hackers,” seems to be severely delusional. And yet, the two statements are logically equivalent.

What are we to make of this?

This is called the problem of the “ontological commitment of the existential quantifier,” and it goes quite a bit further than that. “Ontological” here means “related to being,” so, for example, to the actual being, the existence, of unicorns.

When I say “unicorns exist,” I am making an ontological commitment, I am saying that these animals are actually standing around somewhere, presumably waiting for a hacker to come along. When I say instead that not all things are non-unicorns, I seem to be saying something about a property of existing things (all those non-unicorns which nobody disputes), but I’m not directly claiming that unicorns exist.

If you think that all this is nuts and a total waste of time, personally I tend to agree. Still, it’s a big part of what philosophers do …

Read the full article which is published on Daily Philosophy (external link)