  Central to Autrecourt’s teaching is the view that all evident knowledge (with the exception of the certitude of faith) must be reducible to the first principle (primum principium), i.e., to the principle of non-contradiction. An inference yields evident knowledge only when the affirmation of its antecedent and the negation of its consequent are contradictory. This means that the antecedent and the consequent, or, more precisely, what is signified by the antecedent and the consequent, must be identical, “because if this were not the case, it would not be immediately evident that the antecedent and the opposite of the consequent cannot stand together without contradiction.” It is in the context of this theory that Autrecourt launches an attack on our claim to have certain knowledge of the existence of substances and causal relations. If A and B are two distinct entities, he says, one cannot infer with certainty (knowledge of) the existence of A from that of B or vice versa, for the affirmation of the one and the denial of the other does not result in a contradiction. On the basis of this principle, one may not infer (knowledge of) the existence of effects from knowledge of their causes, nor (knowledge of) the existence of substances from knowledge of their accidents. Autrecourt’s theory about the evidentness of inferences was contested by his contemporaries and should be understood in the light of the late-medieval theory of consequences.