Among challenges to libertarians, the Mind Argument has loomed large. Believing that this challenge cannot be met, Peter van Inwagen, a libertarian, concludes that free will is a mystery. Recently, the Mind Argument has drawn a number of criticisms. Here I seek to add to its woes. Quite apart from its other problems, I argue, the Mind Argument does a poor job of isolating the important concern for libertarians that it raises. Once this concern has been clarified, however, another argument serves to renew the challenge. The Assimilation Argument challenges libertarians to explain how ostensible exercises of free will are relevantly different from other causally undetermined outcomes, outcomes that nobody would count as exercises of free will. In particular, libertarians must explain how agents can have the power to settle which of two causally possible futures becomes the actual future. This will require them to distinguish cases where this power is supposedly present from similar cases where it's clearly absent.
