Thanks are justly due for boons unbought.

The line contrasts gifts that are freely given with benefits that are effectively “bought” (through payment, obligation, or self-interest). Ovid’s sentiment is that gratitude is most properly owed when a favor is unpurchased—i.e., when it is offered without coercion, transaction, or expectation of return. Implicitly, it questions the moral status of benefaction that is really a bargain: if a “boon” is paid for, gratitude is less a virtue than a social formality. The quote thus participates in a long classical ethical tradition distinguishing genuine generosity from calculated exchange, and it elevates voluntary kindness as the true ground of thanks.