Cursus Philosophicus Thomisticus, Prima Pars Artis Logicae

From the preceding chapter it is gathered that there is one principle and root from which the defect and invalidity of consequences arise, namely: “Whenever the antecedent can be made true and the consequent false, the consequence is bad.”

For since a consequence is an inferential connection between two propositions, whenever a true antecedent is given without a true consequent, the connection is neither good nor formal, because one can stand without the other, that is, the truth of one without the truth of the other. But if one stands without the other, the connection is dissolved; therefore that connection and consequence is bad, because it can be dissolved. Hence it follows that whatever makes the antecedent more readily true than the consequent leads to a bad consequence, because it makes it possible for the antecedent to stand true without the consequent. Conversely, whatever makes the consequent harder to render false than the antecedent contributes to, or establishes, the goodness of the consequence.

From these principles all the defects of consequences can be elicited, so that one can assign to each consequence the defect from which it suffers. Indeed, Master Soto, in book 5 of the Summulae, assigns seven intrinsic defects and just as many extrinsic defects in syllogisms. He calls those defects intrinsic which belong to the middle; they are called intrinsic because they render the premises incapable. He calls extrinsic those which belong to the extremes in the manner of inferring conclusions, because they fail to observe the logical properties or the proper arrangement in quantity and quality, distribution, etc. Nearly all intrinsic defects reduce to this: either the middle is taken equivocally, or it is not distributed completely or perfectly, or the conditions and rules set out above¹ are not observed, so that the middle is not taken more broadly where it is undistributed than where it is distributed. Finally, extrinsic defects consist in failing to observe all the logical properties in the extremes. These defects will be shown individually in the following chapter.