H.L.A. Hart, answered from the texts and cited to the page.
Legal rules are framed in natural language, and natural-language terms — vehicle, reasonable, fairness, fair rent — have a core of certainty where their application is settled and a penumbra of doubt where it is contested. This is the open texture of language as it appears in legal rules. In the core, the rule guides decision in a fairly determinate way: every time I drive past a 30 mph sign I know the speed at which I ought to be driving.
In the penumbra, the rule does not by itself settle the case, and the judge must go beyond the rule to consult standards of policy, morality, and equity. In doing so, the judge in some measure makes the law that she applies. Judicial discretion is therefore real and inevitable in hard cases.1 The choice the judge makes is not capricious; it is guided by precedent, by principle, by the values implicit in the legal system; it must be defended by reasons that fit the rest of the law and serve recognised legal values.
But it is not, as Dworkin claimed, the discovery of a uniquely correct interpretive answer that already lay hidden in the materials. People inveterately disagree about the relative weight of competing principles; political communities embed conflicting commitments in their legal systems; there is no Archimedean standpoint from which the right priorities can be settled. The judge's task is to settle a priority, not to find one that was always there.234
On this interpretive view, the law is never incomplete or indeterminate, so the judge never has occasion to step outside the law and exercise a law-creating power in order to reach a decision. It is therefore to such implicit principles, with their moral dimensions, that courts should turn in those 'hard cases' where the social sources of the law fail to determine a decision on some point of law.The Concept of Law, pp. 273–274
It is of course true that an important function of the rule of recognition is to promote the certainty with which the law may be ascertained. This it would fail to do if the tests which it introduced for law not only raise controversial issues in some cases but raise them in all or most cases.The Concept of Law, pp. 251–252
Essay 3 treats the indeterminacy of legal rules as if this was always a purely linguistic matter, that is, solely a function of the indeterminacy of the constituent words used in the fonnulation of a particular rule.Essays in Jurisprudence and Philosophy, p. 14
The open texture of law means that there are, indeed, areas of conduct where much must be left to be developed by courts or officials striking a balance, in the light of circumstances, between competing interests which vary in weight from case to case.The Concept of Law, pp. 135–136
The chapter 7 doctrine has been the central battlefield of the Hart-Dworkin debate. Dworkin's "The Model of Rules" (1967) and Taking Rights Seriously (1977) advanced the strong-discretion / one-right-answer critique; Law's Empire (1986) developed the interpretive theory of law as constructive interpretation. The legal-realist charge from the other flank — that "law" is no more than what officials in fact do — I treated as overstated in The Concept of Law. The Postscript acknowledges that the chapter 7 account understated the role of principles but holds the line that interpretive discretion is genuine discretion. MacCormick's Legal Reasoning and Legal Theory (1978) developed the analysis further and is broadly compatible with the Postscript position.