אתיקה, חלק IV, משפט 72

If it be asked: What should a man's conduct be in a case where he could by breaking faith free himself from the danger of present death? Would not his plan of self--preservation completely persuade him to deceive? This may be answered by pointing out that, if reason persuaded him to act thus, it would persuade all men to act in a similar manner, in which case reason would persuade men not to agree in good faith to unite their forces, or to have laws in common, that is, not to have any general laws, which is absurd.