Monarchia (II, x, 4)

(4) Dico ergo quod, si romanum Imperium de iure non fuit, Cristus nascendo persuasit iniustum; consequens est falsum: ergo contradictorium antecedentis est verum. Inferunt enim se contradictoria invicem a contrario sensu. (4) I say therefore that if the Roman empire was not based on right, Christ by his birth assented to an injustice; the consequent is false; therefore the contradictory of the antecedent is true. For contradictory statements are mutually exclusive: if one is false, the other must be true.