“Ignorance of the law is no excuse,” says an ancient legal nostrum.

The reason for it is simple enough: If ignorance could excuse any crime, then ignorance might excuse every crime. Rather than impose on the legal system the obligation to prove a defendant’s knowledge of the law – with the defendant’s incentive running in the opposite direction, toward ignorance of the law – the legal system assigns to itself no obligation in this regard. Upon which defendants find that they have an enormous incentive to know what the law is and to comport themselves accordingly.

This is not necessarily as crazy as it sounds. It doesn’t assume that the accused did know the law; no one knows all the laws. No one ever has. It simply says that ignorance doesn’t matter.

How can that be defensible? In the old days, it wasn’t so difficult. As Cicero wrote:

There is a true law, a right reason, conformable to nature, universal, unchangeable, eternal, whose commands urge us to duty, and whose prohibitions restrain us from evil… This law cannot be contradicted by any other law, and is not liable either to derogation or abrogation. Neither the senate nor the people can give us any dispensation for not obeying this universal law of justice. It needs no other expositor and interpreter than our own conscience. It is not one thing at Rome and another at Athens; one thing to–day and another to–morrow; but in all times and nations this universal law must for ever reign, eternal and imperishable.

Everyone can know the universal law by use of right reason. And by means of living at Rome or Athens, say, a citizen could become habituated to local modes of conduct, and these would supply whatever content the natural law did not: Much manmade law is merely customary, and this type of law is neither an instantiation of the natural law nor a violation of it. Still, though, some customs may be needed to facilitate commerce and peace.

To learn these customs, whenever an individual entered into a new line of work, or into any other new endeavor, we could expect that he would observe carefully the actions of others who were already engaged in it; in general, he would conform himself to their pattern. In the process he would come to appreciate the inner logic of being a ropemaker at Athens, before he attempted to make and sell rope at Athens himself.

So while it is impossible for any individual, even a lawyer or a judge, to sit down and write out the entire law code, it could still be relatively defensible to hold individuals accountable for their violations of it. The law was an expression of two things that individuals ought to know, namely the universal law and the particulars of one’s own time and place.

But this is in a sense a house of cards: What if custom and the universal law conflict? Which one prevails? Cicero says that it’s the custom that must give way, but it is not clear that it actually will give way in any particular instance. Bad customs can endure for centuries, sometimes. Or what if the written law is ambiguous, as applied to a particular case? What if one law conflicts with another, or if there is a dispute over which part of a large customary legal code governs a particular circumstance? What if two parts of universal law appear to conflict with one another? Or if there is a question about which part of universal law is applicable in a particular case? To resolve these types of conflicts is the work of a judiciary.

As F. A. Hayek put it, “The task of the judge will be to tell [the parties in a dispute] what ought to have guided their expectations, not because anyone had told them before that this was the rule, but because this was the established custom which they ought to have known.” (Law, Legislation, and Liberty vol I p 87) And also: “The judge… serves, or tries to maintain and improve, a going order which nobody has designed, an order that has formed itself without the knowledge and often against the will of authority, that extends beyond the range of deliberate organization on the part of anybody, and that is… based on… [individuals’] expectations becoming mutually adjusted.” (LLL I, pp 118-119).

Yet even with a judiciary, the problems are not fully resolved. One could say, with Hayek, that the purpose of the judge is to bring about the gradual reconciliation of customs. Or one could say, more ambitiously, that customs are eventually to be reconciled with unviersal law. But either claim concedes the point: The laws remain in conflict for now. As a result, a fully formed Hayekian legal order would presumably have to incorporate some institutions of mercy as well, to account for cases in which the reconciliation of the laws, one to another, might seem in the process to have treated individuals with excessive harshness. Two such institutions of mercy suggest themselves from experience: executive clemency and the equity courts, at least in their early history.

And so things might go, except for one difficult fact: Modern law is based on the rule of experts.

Modern law is not based on custom or on attempts by an ongoing community to establish universal norms or to divine universal laws. It is based on the idea that a particular science, known only to a few, and accessible in principle only to them, should guide the state.

In this respect, modern law draws on a tradition more ancient than Cicero, but not, I think, a better one. Modern legislators make an enormous number of rules, and they delegate much rulemaking power to the executive, which makes many more. The entire process is guided by those who purportedly know better.

The problem with this arrangement is simple. Many if not all particular aspects of a grown legal order can be reliably inferred by ordinary people within it. The findings of expert planners can not be so inferred. That’s definitional: The expert’s authority rests on others not having the same insight that he does, and on the expert insight being set off from common sense.

It’s not merely a matter of being a ropemaker at Athens, and practicing a trade in ways similar to others. Rather, it’s about having data that no ropemaker ever had, and using that data to regulate the practice of ropemakers. This causes all sorts of problems, even in cities that aren’t Athens. Consider this example from New York City:

According to the vast majority of police departments and district attorneys in New York State — not to mention knife manufacturers, labor unions, and almost everyone else who knows a thing about knives — what [Richard] Neal was carrying was a perfectly legal folding knife. When gravity knives were banned under New York State law in the 1950s, the legislature actually had a very specific style of weapon in mind — a foot-long terror that bears no resemblance to a knife like the one Neal had. True gravity knives, for all intents and purposes, have been extinct for the better part of a century; today they’re relegated mostly to the antiques section on eBay.

Nonetheless, under the department’s unique interpretation of Penal Code 265.01, almost every pocketknife on the market today can be considered a gravity knife. It’s as if authorities in New York City were using an antiquated law against flintlock muskets to prosecute BB-gun owners.

And the prohibition is as strict as it is all-encompassing. A knife that can be shoehorned into that definition is not only illegal to carry, it’s illegal to possess at all, even within one’s home. The only narrow exceptions apply to those “actively engaged” in hunting and fishing, and are essentially meaningless in New York City.

So, because at one time in the 1950s, street gangs – quaintly, almost touchingly – favored a particular type of knife, workmen today go to jail. This and countless other laws like it make a mockery of the citizens’ responsibility to know, or at least to reliably infer, the law. It is undoubtedly not a bad thing that the Federal Register requires its expert findings to be made public, but it hardly helps when these findings are so voluminous and detailed that they defy comprehension. The power to infer the laws, and perhaps the moral obligation to obey them, disappears; the rule of experts is flatly incompatible with this conception of justice.