Why free will remains a mystery

Peter van Inwagen contends that free will is a mystery. Here I present an argument in the spirit of van Inwagen's. According to the Assimilation Argument, libertarians cannot plausibly distinguish causally undetermined actions, the ones they take to be exercises of free will, from overtly randomized outcomes of the sort nobody would count as exercises of free will. I contend that the Assimilation Argument improves on related arguments in locating the crucial issues between van Inwagen and libertarians who hope to demystify free will, while avoiding objections these arguments have faced