Lon Fuller lists eight principles of what he terms the ‘principles of legality’, or the requirements for the rule of law. These are the requirements of generality, promulgation, non-retroactivity, clarity, non-contradiction, possibility of compliance, constancy through time and congruence between official action and declared rule (The Morality of Law, 1969, Yale: Yale UP). ‘Non-retroactivity’ – or non-retrospectivity, as it will be referred to in this chapter – is an important requirement of the rule of law since individuals cannot be expected to abide by the laws of a democratic society unless they are in a position to know what those laws are. Above all, the rule of law demands nonretroactivity in the criminal sphere; in other words, laws should not be passed to criminalise past activities which were innocent at the time they were carried out. The principle of non-retrospectivity is closely linked with that other pillar of the rule of law, legal certainty. The certainty principle condemns the enactment of excessively vague laws that delegate to administrators the power to deal arbitrarily with the citizen, particularly in criminal law. In Nazi Germany, for example, people could be prosecuted for ‘acts deserving of punishment according to the healthy instincts of race’ and, in the Soviet Union, criminal charges could be brought under the Soviet Criminal Code which prohibited all ‘socially dangerous acts’. In a sense, the rule against retrospectivity is a sub-set of the requirement of legal certainty, since you cannot know what your liability under the law is until that law has been properly formulated.