Stuck on Existence

Relevant to my meanderings is this post Kevin wrote about a year ago:

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.

But could not the concept of X exist, as a concept, but X itself not (yet) exist and would thus need to be brought into existence?