The phrase '''Nulla poena sine lege''' (
Latin: "no penalty without a law") refers to the
legal principle that one cannot be punished for doing something that is not prohibited by law. This principle is accepted as just and upheld by the
penal codes of
constitutional states, including virtually all modern
democracies. It is related to the principle called "''
Nullum crimen, nulla poena sine praevia lege poenali''", which means penal law cannot be enacted retroactively.
One complexity is the lawmaking power of judges under
common law. Even in
civil law systems that do not admit judge-made law, when does the function of interpretation of the criminal law end and judicial lawmaking begin?
The question of
jurisdiction may sometimes come to contradict this principle. For example,
customary international law allows the prosecution of
pirates by any country (applying
universal jurisdiction), even if they did not commit crimes at the area that falls under this country's law. A similar principle has appeared in the recent decades with regard to crimes of
genocide (see
genocide as a crime under domestic law); and
UN Security Council Resolution 1674 "reaffirms the provisions of paragraphs 138 and 139 of the 2005
World Summit Outcome Document regarding the responsibility to protect populations from genocide, war crimes, ethnic cleansing and crimes against humanity"
[1] even if the State in which the population is being assaulted does not recognise these assaults as a breach of domestic law. However, it seems that universal jurisdiction is not to be expanded substantially to other crimes, so as to satisfy ''Nulla poena sine lege''.
The argument has been proposed that this exercise does not violate nulla poena sine lege, since these acts, even if not prohibited under the law of any country, are in violation of international law, which many legal theorists view as being equally law. However, this view depends on accepting as law mere intent, presumption and personal preference, rather than something that has been formally codified, which is a step many legal practicians are not quite prepared to make, all theories aside.
Natural law theorists or
divine command theorists would further add that nulla poena sine lege is not violated if the punished act is against natural law or the law of God, respectively, even if it violates no positive law.
See also
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Nullum crimen, nulla poena sine praevia lege poenali
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Ex post facto law