Non faciat malum, ut inde veniat bonum - "Not to do evil that good may come." Performing some illegal action is not excused by the fact that a positive result came therefrom. Often used to argue that some forms of expression, such as graffiti or pornographic films, cannot be given the protection of law (e.g. copyright) as they are or may be considered illegal or morally reprehensible.⏎
Quod est necessarium est licitum - What is necessary is lawful.⏎
Nullum crimen, nulla poena sine praevia lege poenali - There can be neither crime nor punishment unless there is a penal law first.⏎
Pacta sunt servanda - Agreements must be kept. Contracts are the law or contracts establish obligations (between those who sign them).⏎