Primum est non nocere.
First of all, do no harm. (Hippocrates; The maxim has become an ethical guiding principle in medicine.)
Nihil agere delectat.
It is pleasant to do nothing. (Cicero, De oratore)
Iniuria non excusat iniuriam.
One wrong does not justify another. (N/A)
Silent enim leges inter arma.
Laws are silent in times of war. (Cicero, Pro Milone.)
Vivere est cogitare.
To live is to think. (Cicero, Tusculanae disputationes)
Beneficium accipere libertatem est vendere.
To accept a favour is to sell freedom. (Publilius Syrus)
Accipere quam facere praestat injuriam.
It is better to suffer an injustice than to do an injustice. (Cicero, Tusculanae disputationes)
Commodum ex iniuria sua nemo habere debet.
No person ought to have advantage from his own wrong. (N/A)
Certum est, quia impossibile.
It is certain, because it is impossible. (Tertullianus, De carne Christi. Later in the form Credo, quia absurdum -- I believe, although it is absurd.)
A fonte puro pura defluit aqua.
From a pure spring flows pure water. (N/A)