Law demands an absolute subjection to its rules and commands.
Law has enforcing authority derived from state.
It is heteronymous (being imposed upon men upon the outer life of men).
Law regulates mens relations with others and with society.
A promissory note is in force for three years. It is the debtors duty to repay the loan. It is the legal duty. The creditor can enforce a legal action against the debtor within three years from the date of execution of promissory note before the court of law.
Morality demands that men should act from a sense of ethical duty.
Morality has no such enforcing authority from state.
It is autonomous (coming from the inner life of men).
It governs the inner life of men.
If the promissory note is time-barred, then the legal duty of the debtor turns into moral duty. Of course, moral duty is not enforceable before the court of law.
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Law does not administer morality or politeness. Morality is the cultural lesson which every society has its own. Law only prohibits a person to inflict pain on others ; so far as you are not harmful to others, your immorality may not be within the purview of Law.