The incompatibility and evident opposition of two ideas which are the subject of one and the same proposition.
In general, when a party accused of a crime contradicts himself, it is presumed he does so because he is guilty, for truth does not contradict itself and is always consistent, whereas falsehood is generally inconsistent and the truth of some known facts will contradict the falsehood of those which are falsely alleged to be true. But there must still be much caution used by the judge, as there may be sometimes apparent contradictions which arise either from the timidity, the ignorance or the inability of the party to explain himself, when in fact he tells the truth.
When a witness contradicts himself as to something which is important in the case, his testimony will be much weakened, or it may be entirely discredited and when he relates a story of facts which he alleges passed only in his presence, and he is contradicted as to other facts which are known to others, his credit will be much impaired.
When two witnesses or other persons, state things directly opposed to each other, it is the duty of the judge or jury to reconcile these apparent contradictions. When this cannot be done, the more improbable statement must be rejected or, if both are entitled to the same credit, then the matter is as if no proof had been given.