At one point, Hart identifies legal positivism with the evaluative argument is, of course, at the heart of legal philosophy in general. No legal philosopher can only be a legal positivist. A full understanding of the law also requires accountability for what might be considered the merits of the law (should the law be effective or elegant and fair?); what role should the law play in the judicial system (should valid law always be applied?); what claim does the law have to our obedience (is there a duty to obey?); And also the more practical questions of what laws we should have and whether we should be right. Legal positivism does not seek to answer these questions (although Murphy 2014:88-108 to argue that the theory has important first-rate implications for legal practice). Nevertheless, positivism`s assertion that the existence and content of law depend only on social facts gives shape to them. The second thesis is important while maintaining the conceptual separation between law and morality. That is, this separation thesis draws the line between what the law is and what the law should be. Since there is no necessary connection between law and morality, morality plays no role in determining what the law is. He can illustrate the philosophical stakes of legal positivism by comparing it to a number of other theses with which he is sometimes wrongly identified, and not only by his opponents (see also Hart 1958, Führer 1996 and Schauer 1996). The most influential criticisms of legal positivism all stem in one way or another from the suspicion that it does not give morality the legitimacy it deserves. A theory that emphasizes the factuality of law seems to contribute little to our understanding that law has important functions in ensuring the proper functioning of human life, that the rule of law is a cherished ideal, and that the language and practice of law are highly moralized. Accordingly, critics of positivism argue that the most important features of law are not to be found in its source-based character, but in the ability of law to promote the common good, protect human rights, or govern with integrity. The law does not necessarily meet the conditions under which it is adequately evaluated (Lyons 1984:63; Hart 1961 [2012: 185-186]].
The law must be just, but it must not be; it should promote the common good, but sometimes it does not; It was supposed to protect moral rights, but it can fail miserably. This is what we can call the thesis of moral fallibility. The thesis is correct, but it is not the exclusive property of positivism. Thomas Aquinas accepts it, Fuller accepts it, Finnis accepts it, and Dworkin accepts it. Only a blatant misunderstanding of ideas such as Thomas Aquinas` assertion that “an unjust law does not seem to be a law at all” could suggest otherwise. The law can be essentially moral in character while being morally deficient. Even if each law always delivers justice (formal justice; Justice according to the law), this does not mean that he does justice. Even if every law has a prima facie right to be applied or followed, it does not follow that it has such a right. The gap between these partial and conclusive judgments is all that a theory of natural law needs to adapt to the fallibility thesis. It is sometimes said that positivism more surely grasps the fallibility of law, because once we see that it is a social construct, we will be less likely to pay undue tribute and be more willing to make a clear moral assessment of the law. This claim appealed to several positivists, including Bentham and Hart. But while this may stem from the truth of positivism, it cannot provide an independent argument for it.
If the law is essentially moral in character, then it is obscured, not clarifying, to describe it as a source-based governmental structure. In reality, however, legal positivism cannot be identified with both theses: both are false. There are many necessary “connections,” trivial and non-trivial, between law and morality. As John Gardner notes, legal positivism takes a position on only one of them; it rejects any dependence on the existence of a right on its merits (Gardner 2001). And with regard to this relationship of dependence, legal positivists are concerned with much more than the relationship between law and morality, because in the sole sense that they insist on a separation of law and morality, they must also insist – and for the same reasons – on a separation of law and economics. There is no doubt that moral and political considerations influence the philosophy of law. As Finnis says, the reasons we have for establishing, maintaining, or reforming the law include moral reasons, and so these reasons shape our conceptions of law (1980 [2011:266-273] and 1996:204). But what concepts? If one accepts, like Finnis, that the existence and content of law can be identified without resorting to moral arguments, and that “human law is artifact and artificiality; and no conclusions from moral premises” (1996:205), it becomes difficult to see how the theory of natural law he developed rivals the truth of legal positivism rather than absorbs it (see Gardner 2001, 225-227). It also distorts Lon Fuller`s criticism of Hart (Fuller 1958 and 1964). Fuller has two main points. First, he believes that it is not enough for a legal system to be based on the usual social rules, since the law could not guide behaviour without being at least minimally clear, consistent, public, forward-looking, etc.
– that is, without showing, to some extent, those virtues collectively referred to as the “rule of law”. Suffice it to say that this is compatible with the law of sources. Even if the moral characteristics were identical or superimposed on these constitutional characteristics, they do so because of their normative character. Whatever virtues may be contained in clear, consistent, forward-looking and open practices, one can find not only in the law, but in all other social practices with these characteristics, including custom and positive morality. And such virtues, if they exist, are small: there is little, if any, that speaks of a clear, consistent, forward-looking, public and impartially administered system of racial segregation, for example. Fuller`s second concern is that if the law is a fact, we have no explanation for the duty of obedience. He asks how “an amoral date called a law could have the special property of creating an obligation to obey it” (Fuller 1958:656). One possibility he overlooks is that this is not the case. But even if Fuller is right in his undisputed assumption, the “particular quality” he doubts is a familiar feature of many practices. Compare promises: Whether a company has a promise practice and what someone has promised is a matter of social fact. But promise creates moral obligations of performance or compensation. An “amoral date” may indeed play a role, along with other premises, in a valid argument for moral conclusions.
Many other philosophers, also encouraged by the title of Hart`s famous essay “Positivism and the Separation of Law and Morals” (1958), treat theory as a denial that there is a necessary connection between law and morality – they must be “separable” in a sense, but not actually separate (Coleman 1982).