"I think therefore I am." It's hard to dispute either the argument or the conclusion. But while I undoubtedly exist, do I have to be one of the fundamental objects in the ontology?
Here is a line of thought to that conclusion, somewhat similar to some things I've heard Rob Koons say. Non-fundamental objects are entia rationis, at least in part creatures of our cognitive organization of the world. But we cannot be, even in part, mere creatures of our cognitive organization of the world on pain of circularity. So whatever non-fundamental objects there may be, we are not among them.
I think the controversial claim in the argument may be that non-fundamental entities are entia rationis, but I am not sure. This whole line of argument is difficult for me to think about.