Duty

In economics, a duty is a kind of tax; often associated with customs.

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Duty in ethics, in a job, or simply after making any agreement about who does what, expresses that which one must do -- an obligation. Religious and political establishments (structures in the exercise of power) have long found that internalizing control comes cheaper than close surveillance. In the event of default, agents scattered through society stand ready to remind the recalcitrant of their "duty" to monarch, lord, country or church. Such injunctions often adopt a moral tone.