Is Stephen Darwall right about why moral obligations are second-personal?

Like most philosophers, Stephen Darwall believes that metaethical truths are not grounded in substantive morality. In response, I contend that Darwall’s second-personal account of moral obligation is a first-order moral thesis, despite appearances (and Darwall’s own protestations) to the contrary. My case is made by showing that Darwall’s theory is incompatible with at least one coherent, though incorrect, moral thesis. My argument lends support to the view that all metaethical theses are substantively moral.