[In 1957, Professor H.L.A. Hart of Oxford University gave a lecture at Harvard, subsequently published as "Positivism and the Separation of Law and Morals, 71 Harv. J. Rev. 593 (1958). In this talk, Hart argued as follows, 71 Harv. L. Rev. at 607:
 

A legal rule forbids you to take a vehicle into the public park. Plainly this forbids an automobile, but what about bicycles, roller skates, toy automobiles? What about airplanes? Are these, as we say, to be called "vehicles" for the purpose of the rule or not? If we are to communicate with each other at all, and if, as in the most elementary form of law, we are to express our intentions that a certain type of behavior be regulated by rules, then the general words we use - like "vehicle" in the case I consider - must have some standard instance in which no doubts are felt about its application. There must be a core of settled meaning, but there will be, as well, a penumbra of debatable cases in which words are neither obviously applicable not obviously ruled out.

Professor Lon Fuller's response, printed back-to-back with Hart's article, contains one of the most well-known statements of the importance of purposive interpretation.]

THE PROBLEM OF INTERPRETATION: THE CORE AND THE PENUMBRA

It is essential that we be just as clear as we can be about the meaning of Professor Hart's doctrine of "the core and the penumbra," because I believe the casual reader is likely to misinterpret what he has to say. Such a reader is apt to suppose that Professor Hart is merely describing something that is a matter of everyday experience for the lawyer, namely, that in the interpretation of legal rules it is typically the case (though not universally so) that there are some situations which will seem to fall rather clearly within the rule, while others will be more doubtful. Professor Hart's thesis takes no such jejune form. His extended discussion of the core and the penumbra is not just a complicated way of recognizing that some cases are hard, while others are easy. Instead, on the basis of a theory about language meaning generally, he is proposing a theory of judicial interpretation which is, I believe, wholly novel. Certainly it has never been put forward in so uncompromising a form before.

As I understand Professor Hart's thesis (if we add some tacit assumptions implied by it, as well as some qualifications he would no doubt wish his readers to supply) a full statement would run something as follows: The task of interpretation is commonly that of determining the meaning of the individual words of a legal rule, like "vehicle" in a rule excluding vehicles from a park. More particularly, the task of interpretation is to determine the range of reference of such a word, or the aggregate of things to which it points. Communication is possible only because words have a "standard instance," or a "core of meaning" that remains relatively constant, whatever the context in which the word may appear. Except in unusual circumstances, it will always be proper to regard a word like "vehicle" as embracing its "standard instance," that is, that aggregate of things it would include in all ordinary contexts, within or without the law. This meaning the word will have in any legal rule, whatever its purpose. In applying the word to its "standard instance," no creative role is assumed by the judge. He is simply applying the law "as it is."

In addition to a constant core, however, words also have a penumbra of meaning which, unlike the core, will vary from context to context. When the object in question (say, a tricycle) falls within this penumbral area, the judge is forced to assume a more creative role. He must now undertake, for the first time, an interpretation of the rule in the light of its purpose or aim. Having in mind what was sought by the regulation concerning parks, ought it to be considered as barring tricycles? When questions of this sort are decided there is at least an "intersection" of "is" and "ought," since the judge, in deciding what the rule "is," does so in the light of his notions of what "it ought to be" in order to carry out its purpose.

If I have properly interpreted Professor Hart's theory as it affects the "hard core," then I think it is quite untenable. The most obvious defect of his theory lies in its assumption that problems of interpretation typically turn on the meaning of individual words. Surely no judge applying a rule of the common law ever followed any such procedure as that described (and, I take it, prescribed) by Professor Hart; indeed, we do not normally even think of his problem as being one of "interpretation." Even in the case of statutes, we commonly have to assign meaning, not to a single word, but to a sentence, a paragraph, or a whole page or more of text. Surely a paragraph does not have a "standard instance" that remains constant whatever the context in which it appears. If a statute seems to have a kind of "core meaning" that we can apply without a too precise inquiry into its exact purpose, this is because we can see that, however one might formulate the precise objective of the statute, this case would still come within it.

Even in situations where our interpretive difficulties seem to head up in a single word, Professor Hart's analysis seems to me to give no real account of what does or should happen. In his illustration of the "vehicle," although he tells us this word has a core of meaning that in all contexts defines unequivocally a range of objects embraced by it, he never tells us what these objects might be. If the rule excluding vehicles from parks seems easy to apply in some cases, I submit this is because we can see clearly enough what the rule "is aiming at in general" so that we know there is no need to worry about the difference between Fords and Cadillacs. If in some cases we seem to be able to apply the rule without asking what its purpose is, this is not because we can treat a directive arrangement as if it had no purpose. It is rather because, for example, whether the rule be intended to preserve quiet in the park, or to save carefree strollers from injury, we know, "without thinking," that a noisy automobile must be excluded.

What would Professor Hart say if some local patriots wanted to mount on a pedestal in the park a truck used in World War II, while other citizens, regarding the proposed memorial as an eyesore, support their stand by the "no vehicle" rule? Does this truck, in perfect working order, fall within the core or the penumbra?

Professor Hart seems to assert that unless words have "standard instances" that remain constant regardless of context, effective communication would break down and it would become impossible to construct a system of "rules which have authority." If in every context words took on a unique meaning, peculiar to that context, the whole process of interpretation would become so uncertain and subjective that the ideal of a rule of law would lose its meaning. In other words, Professor Hart seems to be saying that unless we are prepared to accept his analysis of interpretation, we must surrender all hope of giving an effective meaning to the ideal of fidelity to law. This presents a very dark prospect indeed, if one believes, as I do, that we cannot accept his theory of interpretation. I do not take so gloomy a view of the future of the ideal of fidelity to law.

An illustration will help to test, not only Professor Hart's theory of the core and the penumbra, but its relevance to the ideal of fidelity to law as well. Let us suppose that in leafing through the statutes, we come upon the following enactment: "It shall be a misdemeanor, punishable by a fine of five dollars, to sleep in any railway station." We have no trouble in perceiving the general nature of the target toward which this statute is aimed. Indeed, we are likely at once to call to mind the picture of a disheveled tramp, spread out in an ungainly fashion on one of the benches of the station, keeping weary passengers on their feet and filling their ears with raucous and alcoholic snores. This vision may fairly be said to represent the "obvious instance" contemplated by the statute, though certainly it is far from being the "standard instance" of the physiological state called "sleep."

Now let us see how this example bears on the ideal of fidelity to law. Suppose I am a judge, and that two men are brought before me for violating this statute. The first is a passenger who was waiting at 3 A.M. for a delayed train. When he was arrested he was sitting upright in an orderly fashion, but was heard by the arresting officer to be gently snoring. The second is a man who had brought a blanket and pillow to the station and had obviously settled himself down for the night. He was arrested, however, before he had a chance to go to sleep. Which of these cases presents the "standard instance" of the word "sleep"? If I disregard that question, and decide to fine the second man and set free the first, have I violated a duty of fidelity to law? Have I violated that duty if I interpret the word "sleep" as used in this statute to mean something like "to spread oneself out on a bench or floor to spend the night, or as if to spend the night"?