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In economics, a duty is a kind of tax; often associated with customs. The muost common example is Excise.
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.
That a father and his children have mutual duties implies that there are moral laws regulating their relationship; that it is the duty of a servant to obey his master within certain limits is part of a definite contract, whereby he becomes a servant engaging to do certain things for a specified wage.
From the root idea of obligation to serve or give something in return, involved in the conception of duty, have sprung various derivative uses of the word; thus it is used of the services performed by a minister of a church, by a soldier, or by any employee or servant.
Properly a "duty" differs from a "tax" in being levied on specific commodities, transactions, estates, andc., and not on individuals; thus it is right to talk of import-duties, excise-duties, deathor succession-duties, andc., but of income tax as being levied on a person in proportion to his income.