T 3.2.6.10

Nor do I perceive how I can easily be mistaken in this matter. I see evidently, that when any man imposes on himself general inflexible rules in his conduct with others, he considers certain objects as their property, which he supposes to be sacred and inviolable. But no proposition can be more evident, than that property is perfectly unintelligible without first supposing justice and injustice; and that these virtues and vicesmoral duties are as unintelligible, unless we have motives, independent of the morality, to impel us to just actions, and deter us from unjust ones. Let those motives, therefore, be what they will, they must accommodate themselves to circumstances, and must admit of all the variations, which human affairs, in their incessant revolutions, are susceptible of. They are consequently a very improper foundation for such rigid inflexible rules as the laws of nature; and 'tis evident these laws can only be deriv'd from human conventions, when men have perceiv'd the disorders that result from following their natural and variable principles.