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6-1 Models

6.91The letter and the spirit of a rule

Thinking tools

A rule's words are a short stand-in for its purpose, and in cases its authors never pictured, following the words and following the purpose give different answers.

H. L. A. Hart's "no vehicles in the park" example in Positivism and the Separation of Law and Morals, and Lon Fuller's reply in the same issue of the Harvard Law Review (volume 71, February 1958).

  1. 1Read

    Hart's example is a legal rule that forbids taking a vehicle into a public park. It plainly forbids a car, he wrote, but what about bicycles, roller skates, toy cars or an aeroplane? Every general word has a core of settled cases and a fringe of doubtful ones, which he called the penumbra, and in the penumbra the words cannot decide for you: someone has to choose, and the choice cannot be pure deduction. Fuller answered that even the core depends on what the rule is for. Suppose local patriots want to mount a World War II truck, in perfect working order, on a pedestal in the park as a memorial. It is certainly a vehicle, and certainly not what the rule was written to stop. His second case was a five-dollar fine for sleeping in a railway station: a passenger dozing upright while he waits at 3 am for a delayed train breaks the words, and a man arrested with his blanket and pillow before he fell asleep breaks only the purpose. The prediction is that any rule applied to enough cases will meet one where its words and its purpose split.

    It happens because a rule is compressed. Its authors write for the cases they can picture, and, as Hart put it, invention and nature keep throwing up variants on the familiar. Software is full of rules in this sense: specifications, linter checks, test suites, access policies and the instructions in a system prompt. A test that passes while the feature is broken is the letter satisfied and the spirit missed. When people do this on purpose, because they are judged by the words, you get Goodhart's law. And reading the spirit needs the purpose, which is Chesterton's fence again: before you set a rule aside as pointless in this case, find out what it was put there to prevent.

    Following the spirit is not always the better choice. The letter gives predictability and fair warning, and an appeal to purpose can be a cover for doing what the reader wanted anyway; Frederick Schauer's 2008 review of the debate notes that Fuller's truck is a weaker example than his sleeper, because it is arguable whether a truck on a pedestal is a vehicle at all. A workable habit is to decide where surprise is costly. In criminal law, contracts and public application programming interfaces (APIs), lean on the letter and change the words when they fail; in a style guide or an internal process, lean on the purpose. Either way, when you find a split, rewrite the rule so the next person does not have to guess.

  2. 2Note

    Example

    McBoyle had been convicted of moving a stolen aeroplane from Ottawa, Illinois, to Guymon, Oklahoma, under the National Motor Vehicle Theft Act of 1919, which covered "any other self-propelled vehicle not designed for running on rails", and sentenced to three years in prison and a 2,000 dollar fine. On 9 March 1931 the US Supreme Court reversed the conviction. Justice Oliver Wendell Holmes wrote that in everyday speech a vehicle calls up the picture of a thing moving on land, that aeroplanes were well known in 1919 but went unmentioned in the debates in Congress, and that a criminal law should give fair warning in words the common world will understand, even when the same policy seems to apply. The law's policy arguably reached a stolen aeroplane too; the court held to the letter. Hart cited the case in his 1958 paper.

  3. 3Apply

    Where it applies

    When a linter, a failing check or a compliance rule blocks you, write down what harm the rule exists to prevent before you either obey it or add an exception. When you write instructions or an evaluation rubric for a language model, put the purpose next to each rule, because the model will meet cases you never pictured and will otherwise follow the words.

  4. 4Think

    Think about it

    Which rule did you follow to the letter recently while knowing it missed its point, and did you tell the person who could change it?

  5. 5Log

    Applied it to something real? Log it.

  6. 6Check
    Test yourselfHart asked whether bicycles and roller skates count under "no vehicles in the park". What did Fuller's war-memorial truck add to the argument?

    That even a case squarely inside the words can fall outside the rule's purpose: a working truck is plainly a vehicle, yet a memorial is not what the rule was for. You cannot apply a rule well without knowing its aim.

  7. 7Go

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  8. 8Source

    Sources and further reading