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To have done something legally wrong within Anglo-American Criminal law systems is to do more than perform a voluntary action that causes a harmful or otherwise immoral state of affairs to exist.  In addition to the voluntary action and causation requirements, Anglo-American criminal law requires that something called the legality requirement be satisfied. Exactly what are the doctrinal expressions of that requirement and what values are served by imposing such a requirement on criminal liability are explored in the readings to follow.  The first selection, amongst so many, are the ‘Principle of Legality’. We will give you a brief overview of the doctrine making up the principle of legality and of the values such doctrines serve.


The principle of legality is in reality a mixture of four values that jointly justify a wide variety of criminal-law doctrines. The values are those of fairness, liberty, democracy, and equality, which are unpacked as follows: It is unfair  to surprise citizens with liability to criminal sanctions when they reasonably relied on their actions not being criminal at the time they were done. It impedes liberty if citizens cannot know the content of the criminal law well enough to take into account the possibilities of penal liability in planning their actions. The value of democratic decision-making requires tat elected legislatures decide what is and what is not criminal (27 CFR § 72.11-MEANING: All Crimes Are Commercial) (not electorally responsive) courts would frustrate that value if they were to take it upon themselves to make conduct criminal without statutory authorization. Equality dictates that those who are in all morally relevant respects alike be treated alike, and this requires that  neither legislatures nor judges single individuals out for arbitrarily  different treatment.  In Anglo-American law, the doctrinal expressions of these values are numerous, and many of such doctrines are over-determined by more than one of the four values just described. The main doctrines are (9) nine in number.

  1. The prohibition against “Common-Law Crimes,” or “Crimes by analogy,” holds that courts are without power to create new crimes, either from whole cloth or by analogy to crimes already prohibited by statue. The primary value furthered by this doctrine is democracy, because the justification for restricting criminal law-making to legislatures over judges. However, fairness and liberty are also served by courts refusing to use (retroactively applicable) adjudication as the occasion on which to announce new crimes.

  2. The doctrine that ex post facto laws are unconstitutional plainly serves the values of fairness and liberty. Retroactive criminal laws would both unfairly disappoint reliance on activity not being  criminal when it is done, and chill liberty by the fear that such surprise might be forthcoming.

  3. The constitutional (Due Process)  prohibition of retroactive judicial enlargement of criminal liability, like the ex post facto limitation on legislation, plainly serves the values of fairness and liberty. For the purposes of these values, it does not matter whether the surprise comes from law-making by a court or by a legislature.

  4. The Void-for-Vagueness doctrine requires legislature to frame acts with sufficient clarity that they can be understood by those to whom they are directed. Such failure of understanding can occur not only because of vagueness in some statutory predicate, but also because of ambiguity in the predicate; inconsistency between this statutory provision and some other; inability of most citizens to be able to verify factually whether the behaviour they contemplate will or will not have the characteristics the statute prohibits. Any of these inabilities to understand makes a criminal statute “Void-for-Vagueness” under the due process clause. The main values served here are fairness and liberty, but the equality value is also served by not allowing vague statutes to grant such discretion to enforcement officials that discriminatory application is made possible.

  5. The prohibition against “bills of attainder” also serves the equality value by requiring of criminal legislation that it be general in the sense that it applied to some class of persons. Such generality is the first step towards equal treatment, because such generality precludes the numerical distinctness of persons being used to single individuals out for criminal punishment. Because bills of attainder are also retroactive in effect, that prohibition also serves the value of fairness and liberty.

  6. The common-law maxim that criminal statutes must be strictly construed serves the values of fairness and liberty by disallowing prosecution in the vague penumbra of some statutes actus reus prohibition. Such a maxim give citizens the benefit of the doubt in the interpretation of criminal statutes, so that any reasonable mistake of interpretation due to the vagueness of the statutory language does not result in criminal liability.

  7. Mistake of law by an accused is allowed as a defense when that defendant's mistake was induced by his reliance on (mis)-advice about the law received from some governmental official. Here, the value is mostly fairness, because the defendant is unfairly whipsawed between one governmental agency (which gave the above advice) and another (the court trying him).

  8. Some state constitutions require that criminal statutes be in English, again with the plain rationale that citizens must be able to find out what is punishable in advance. The values serve are again fairness and liberty.

  9. Mistake of law is also a defense to criminal prosecution in situations where the law making an act criminal was not publicly promulgated. The doctrine also has as its rationale the value of fairness and liberty. What isn’t publicly available cannot be known, so that when applied such a statute unfairly surprises, and the potential for such surprise chills the liberty of those not knowing whether their act is secretly criminal.


The four values, as they coverage to jointly justify these nine doctrines, are what is misleadingly referred to as “the” principle of legality. Such principles prohibits punishment except for actions prohibited by some public, perspective, general, clear, consistent, verifiable, strictly construed, legislatively created law.

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