- Substantive rules of law (such as the rules of torts, contract, and property) are usually assumed to be addressed to two audiences. As conduct rules, the substantive law is addressed to everyone (citizens, officials, and noncitizens). Thus, property law tells us who has dominion over which resources. If this land is mine, then the law communicates the message that I can use my land and exclude others from its use. These very same legal rules also serve as decision rules, they tell courts how to resolve disputes. We usually assume that the content of the conduct rules are the decision rules are identical, but this need not be the case. Professor Meir Dan-Cohen of U.C. Berkeley proposed a very famous thought experiment. He asked us to imagine acoustic separation between ordinary citizens, who would only "hear" the conduct rules, and officials (such as judges), to whom the decision rules would be addressed. This leads naturally to the following thought: should decision rules and conduct rules have the same content or should they differ. And if they differ, how could the law prevent acoustic leakage, e.g. prevent ordinary citizens from learning about the content of the decision rules?
Example? Here's a pretty clear example. Suppose that we have a conduct rule that says, "Ignorance of the law is no excuse." This might be a good conduct rule, because we want citizens to inform themselves about the content of the law, and we certainly don't want citizens deliberately insulating themselves from knowledge of the law in order to create a defense if they charged with its violation. But at the same time, we might prefer that ignorance of the law would serve as an excuse, at least some of the time, when it comes to actually convicting and punishing defendants. Punishment is expensive and injurious, and sometimes no really good purpose will be served by punishing someone who is reasonably ignorant of the law's content.
But how can we excuse ignorance of the law without altering the conduct rule? One way to accomplish this goal would involve some obfuscation by judges. Opinions might state boldly: "Ignorance of the law is no excuse," while simultaneously excusing ignorant defendants on the ground that "knowledge of the legal status of the intentional content is part of the mental state that is an element of the crime." The first formulation is easily accessible to ordinary folks; the second is couched in language that may be opaque except to those trained in the law.