deepest thing I've read all day

From Dr. V.:

If a property is defined as an instantiable entity, then existence cannot be understood as a property of existing particulars. This is because the particular must already exist to be in a position to instantiate any properties including the putative property of existence.

This is a great explanation of the claim "existence is not a predicate."  In John Hick's Philosophy of Religion, it's explained this way (I'm citing this from memory, so this isn't an exact quote):  if we say "a cow exists," we're not saying "a cow has existence," as if existence were a predicate (property):  what we're really saying is "there's an X such that 'X is a cow' is true."  Hick's explanation dovetails perfectly with what Dr. V wrote above.

In slightly simpler language: in order for some X to have (possess) something, X already has to exist.  It therefore makes no sense to say "X has existence" because you're trapping yourself in an infinite loop (or "vicious circle"):  X has to exist before it can be said to have existence.  The very property X is supposed to have must be in place before X can be said to have it.