 He goes on to argue that moral injunctions motivate, and that since only the passions can motivate, moral injunctions must be grounded in the passions. Ultimately, for Hume, what is good is what is useful, a modern version of the Epicureanism, which Burnet had argued, follows from empiricism. For Hume, political constitutions are not grounded in immutable truths, but are based in convention. Since the functioning of a set of conventions depends on people agreeing to abide by certain rules, which are in themselves arbitrary, it is important that people revere authority. For him, the execution of Charles I was a tragedy, brought about by deluded enthusiasts. And he comments in relation to this act: