In response to arguments against the existence of God, and in response to perceived conflicts between divine attributes, theists often face pressure to give up some pretheoretically attractive thesis about the divine attributes. One wonders: when does this unacceptably water down our concept of God, and when is it, as van Inwagen says, ‘permissible tinkering’ with the concept of God? A natural and widely deployed answer is that it is permissible tinkering iff it is does not violate the claim that (...) God is the greatest possible being. Call this the ’perfect being defense.’ In this paper I lay out some influential uses of the perfect being defense, and then argue that this strategy for defending theism fails. (shrink)