 Sensitivity to such matters led Hart to defend a rule-based theory. He says that while sanctions might mark circumstances in which people are obliged to conform, they have an obligation only when subject to a practised social rule requiring an act or omission. The fact that subjects use it as a rule marks it as normative. Three further features distinguish obligation-imposing rules: they must be reinforced by serious or insistent pressure to conform; they must be believed important to social life or to some valued aspect of it; and their requirements may conflict with the interests and goals of the subject (Hart 1961 [1994: 85–88]). This account of the nature of obligations is not an account of their validity. Hart does not say that a legal duty is binding whenever there is a willingness to deploy serious pressure in its support, etc. He holds that a duty is legally valid if it is part of the legal system (i.e., if it is certified as such by the tests for law in that system), and a legal duty is morally valid only if there are sound moral reasons to comply with it. But, at least in his early work, he offers the practice theory as an explanation of duties generally—legal duties are the creatures of legal rules, moral duties of moral rules and so on. (Hart later modified this view, see 1982: 255–268; and 1961 [1994: 256].)