1168. A presumption is an inference drawn from a known fact as to the existence of an unknown fact, which inference the law, in some cases, requires to be drawn and in others permits to be drawn, in the discretion of the Court or Judge. The former class are called legal presumptions.
1169. Legal presumptions obviate the necessity of further proof; but certain of them may be contradicted by other proof. Others cannot be contradicted.
1170. No proof is admitted to contradict a legal presumption, when, on the ground of such presumption, the law annuls certain instruments or disallows a suit, unless the law has reserved the right of giving such proof, and saving what is provided with respect to the oaths or judicial admissions of a party.
1171. The authority of a final judgment (res judicata) supplies a presumption incapable of contradiction in respect of that which has been the object of the judgment, when the demand is founded on the same cause, is between the same parties acting in the same qualities, and is for the same thing as in the action adjudged upon.