Dolus bonus, dolus malus

In a wide sense, the Roman law distinguishes between "good," or rather "permissible" dolus and "bad" or fraudulent dolus. The former is justifiable or allowable deceit; it is that which a man may employ in self-defense against an unlawful attack, or for another permissible purpose, as when one dissembles the truth to prevent a lunatic from injuring himself or others. The latter exists where one intentionally misleads another or takes advantage of another's error wrongfully, by any form of deception, fraud, or cheating.

Mackeld. Rom. Law, § 179 ; Broom, Max. 349 ; 2 Kent, Comm. 560, note.