of nature, for all their usefulness, leave many further opportunities for mutual advantage unrealised (e.g., non-simultaneous cooperative exchanges of labour), unable to overcome the meanness of human nature in the absence of "mutual confidence and security". But unvirtuous individuals will soon learn to cooperate with each other simply from a self-interested expectation of the benefits of future cooperation, and special language is introduced to express one's resolution to perform one's part (on penalty of social distrust)—thus the practice is distinguished from the favors of true friends, and secured through staking one's reputation on faithful performance. The convention is then made moral in the same way as before ("[p]ublic interest, education, and the artifices of politicians") and a fictional act of the mind ("willing an obligation") is fabricated to make sense of the moral obligation. Finally, Hume reinforces this explanation by observing that a promise obligates you even if you mentally crossed your fingers, but does not obligate you if it was honestly unintended or if you were obviously joking, and yet does obligate you if your devious insincerity is apparent to shrewd observers, and yet does not obligate you if induced by force (alone among all motives): "[a]ll these contradictions", Hume says, are best explained by his convention-based account of promising. He adds that the "terrible" Catholic doctrine of intention (viz., that a sacrament is invalidated if its minister is in the wrong state of mind) is actually more reasonable than the practice of promising—since theology is less important than promise-keeping, it can afford to sacrifice utility to consistency. Finally, Hume reviews these "laws of nature" and offers three additional arguments for their artificiality. (1) Justice is commonly defined in terms of property, and yet it is impossible to understand property except in terms of justice. But since there is no natural sentiment of approval for the practice of justice described in neutral language, "abstracting from the notio[n] of property" (e.g., restoring an object to its first possessor), justice is not a natural virtue. (2) Justice and injustice come in bright lines and sharp boundaries, whereas our natural moral sentiments come in degrees. (3) Justice and injustice are universal and general, whereas our natural moral sentiments are partial and particular: e.g., justice might decide in favor of a featherbrained and filthy-rich bachelor instead of a level-headed man trying to support his destitute family, setting aside as irrelevant all the circumstances