Hart holds that law is an instrument of social control. This means that the rules of law must satisfy certain conditions if they are to properly achieve this goal. For instance, citizens may reasonably expect that the rules of law will not be retroactively applied. A principle of fairness is involved here. Citizens should have both the ability and opportunity to obey the law. So, the principle of formal justice, a principle of impartiality, and the principle of fairness are all built into Hart's concept of law. This is a moral beginning, but only a beginning.