Harry Thurston Peck, Harpers Dictionary of Classical Antiquities (1898)

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Bona Fides

A term frequently used by Roman writers, especially by the jurists, and is opposed in meaning to mala fides and dolus malus. It implies the absence of all fraud, insincerity, unfair dealing, and bad faith, and is hence a necessary ingredient in all binding contracts. Bona fide possidere is said of him who has acquired the possession of a thing under what he believes to be a good title.

In various actions arising out of mutual dealings, such as buying and selling, lending and hiring, partnership, and others, bona fides is equivalent to aequum and iustum; and such actions were sometimes called bonae fidei actiones. The formula of the praetor, which was the authority of the iudex, empowered him in such cases to inquire and determine ex bona fide, that is, according to the real merits of the case.