The maxim that ignorance of the law excuses nobody is defended, when it is defended at all, on grounds of administration. If ignorance were an excuse, the argument runs, every defendant would claim it, and the prosecution would be put to proving what was in a mind it cannot enter. That is a good argument as far as it goes, and it does not go as far as the maxim does. It explains why the excuse should be refused in the ordinary run of cases. It does not explain why it is refused in the cases where the ignorance is admitted on every side, is plainly genuine, and could be established without troubling anybody's memory.

Sir Ronald Kessick put the administrative case in its strongest form in 1934 and then unsettled it in a footnote, where he allowed that "the difficulty of proof is a reason for a presumption, not for a rule". A presumption can be displaced by evidence. The maxim admits no evidence at all, and a defendant who proves his ignorance to everyone's satisfaction stands where one who never raised it stands.

The better ground is a different one, and it is rarely stated. Most of what the criminal law forbids is not learned from the criminal law. A man who takes what is not his, or strikes someone who has not struck him, has not been misled by the absence of a handbook. The prohibition is carried by the understanding of the people among whom he lives, and the statute comes afterwards, restating in the language of penalties what was already settled in the language of manners. Where that is so, the plea of ignorance is not false so much as misdirected. What the defendant claims not to have known is not the rule but the reason, and the reason was in front of him.

This account has a consequence that those who hold it do not always welcome. It says nothing whatever about the great body of modern regulation. Whether a return must be filed by the sixth of April, or a licence that covers a van also covers the trailer behind it, are not questions the ordinary understanding settles, and it could not settle them, since the answers are amended faster than anybody could learn them. Here the ignorance really is ignorance of a rule. Nothing in the argument above tells against excusing it.

The courts have noticed, though they have not said so, since saying so would mean giving up the maxim. What they have done instead is to build a set of defences around it, and to insist that a rule cannot be enforced against a person who had no notice of it. Suppose a regulation were made on a Monday, entered in a register that closes at four, and enforced on the Tuesday against a man who is at his work until five. Would the maxim really answer him? When the question has come near a court, the answer has been that the regulation was not yet in force, which is a way of leaving the question alone.

The word doing the work in these decisions is notice, and it is not being used in its everyday sense of having been told. A person has notice, in this line of cases, when the rule was there to be found by somebody in his position who went looking for it. Whether he went looking is not asked, and Mr Justice Harrowby observed that it would be an odd inquiry to begin. The register itself was kept until 1968 in a room above a bank in Chancery Lane, and its keeper reported that it was consulted mainly by students.

What is defensible in the maxim is a narrow claim about one class of prohibitions. What is enforced is a general one, and the space between them has been filled by a doctrine of notice that has never had to be defended in public.