Quod est necessarium est licitum - What is necessary is lawful.⏎
Pacta sunt servanda - Agreements must be kept. Contracts are the law or contracts establish obligations (between those who sign them).⏎
Nulla poena sine lege or nullum crimen, nulla poena sine praevia lege poenali - "No penalty without law" or "no crime, no punishment without a previous penal law". (1) One cannot be prosecuted for doing something that is not prohibited by law. (2) One cannot be prosecuted for doing something that was not prohibited by law at the time and place it was committed, notwithstanding laws made since that time. A form of prohibition on retroactive laws.⏎
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.⏎