According to robert nozick in his book philosophical explanation. We can think of a predicate P as defined the universe U into three region. There are

1) They are: { x: Px is true}

2) They are: { x: Px is false}

3) They are: {x: Px is a categorical mistake}

3 is a bit comfusing, so i will explain. Think of the statement:

Blue taste good

The predicate "taste good" presuppose the set of all foods that we can smell, and blue is not a food at all. It is a categorical mistake to apply blue to the predicate. 3 say that P distinquish the set x, such that Px is a category mistake.


Ok, you ask. So what?

Well, this means when you think about a predicate, you are ontologically commit to 3 classes of things.