Duty

Old French deu (due, owed), from Latin debitus (owed), past participle of debere (to owe) — the same root as debt, debit, and due. Duty entered English in the 14th century meaning an obligation, what is owed — both the payment owed to a government (customs duty, stamp duty) and the moral obligation owed to family, country, or principle. The word is the past participle: something already owed, already obligated, pre-existing the choice.

The concept of duty as moral obligation enters the philosophical record most forcefully through Kant's categorical imperative (Kritik der praktischen Vernunft, 1788): the moral law that commands us to act only according to maxims we could will to be universal laws. Kant's ethics is deontological — duty-based. The rightness of an action depends on whether it fulfills a duty, not on its consequences. Kant's dutiful agent does the right thing because it is right, regardless of outcome.

The commercial duty — tax on goods — has its own history parallel to moral duty. Stamp duty (England, 1694) taxed legal documents; estate duty taxed inherited property; death duty was renamed inheritance tax. Each duty named a specific relationship of owing: the importation of goods owes something to the state; the inheritance of wealth owes something; the execution of a legal document owes something. The state determined what was owed; the merchant, the heir, the lawyer paid it.

Duty-free zones — airports, ships, special economic zones where customs duties are not collected — are defined by the absence of the owed payment. The duty-free shop at Heathrow sells whisky and perfume without the excise and customs that would make them more expensive elsewhere. The duty is absent because you are, technically, in transit — between the obligations of one jurisdiction and another.