Themes such as praising courage, blaming miserliness, honoring generosity and admiring beauty arise commonly in reason and social custom. A poet may express them without appropriating another person's distinctive invention.
Attribution of borrowing must therefore examine not merely the abstract proposition but its particular indications, arrangement and context. The case for borrowing is weaker still when the method of indicating the quality is conventional.
Without this rule, almost every praise poem, elegy or satire would count as a copy of an earlier one, which is not the critical distinction being made.