Full Idea
The principle of legality requires criminal offences to be as precisely defined as possible, so that it can be known with reasonable certainty beforehand what acts are criminal and what are not.
Gist of Idea
The principle of legality requires crimes to be precisely defined in advance of any action
Source
H.L.A. Hart (Law,Liberty and Morality [1963], I 'Conspiracy')
Book Reference
Hart,H.L.A.: 'Law,Liberty and Morality' [OUP 1968], p.12
A Reaction
Hart is discussing a breach of this, where moral judgements are used to condemn something which was not obviously illegal. Families and schools don't have such precise rules, but it seems needed in a vast and pluralistic society.