  Harm-based arguments are nowadays ubiquitous when proposed criminal laws are discussed. Some think this shows that (HPP) is no constraint at all (Harcourt 1999). But it is no surprise that those who merely pay lip service to a principle are not constrained by it. The argument of the previous paragraph was an argument that (HPP) should be followed. To follow that principle is to take seriously the need for an empirical showing—grounded in adequate evidence—that a given law is necessary to prevent a proportionate amount of harm. A better objection is that the error-based argument is incomplete. How widespread would error be if law-makers took themselves to be free of (HPP)? When are the benefits of following (HPP)—in errors prevented—worth the costs—in otherwise impeccable criminal laws? Might there be some other rule that brings us those benefits at a lower cost than (HPP)? We need answers to all these questions, and more, to know if an argument from within non-ideal theory can support (HPP) (Tadros 2016, 94–96).