Possessor in Good Faith

He is deemed a possessor in good faith who is not aware that there exists in his title or mode of acquisition any flaw which invalidates it.

He is deemed a possessor in bad faith who possesses in any case contrary to the foregoing.

Mistake upon a doubtful or difficult a question of law may be the basis of good faith. (Article 526 of the Civil Code)
Good faith is always presumed, and upon him who alleges bad faith on the part of the possessor rests the burden of proof. (Article 527 of the Civil Code)