 Some legal philosophers have responded to this problem by claiming that judges never (or virtually never) have such a choice, and that there is virtually always a right answer to a question of legal rights (Dworkin, 1986a, 1991). Others have responded to the problem by claiming that the law gives judges discretion, in all or some borderline cases, to decide issues that the law does not determine (Hart, 2012, chapter VII.1). That is, the standards of the system leave a choice to judges as to how to decide the issue. Then judges must treat the parties to litigation as having liabilities or obligations or entitlements that were not determinately theirs at the time when the dispute arose. That power of judges appears to some to run contrary to a requirement of the rule of law: that laws (or at least, that legal burdens) should not be imposed retroactively.