Chapter 31.--Use of Dialectics. Of Fallacies.
48. There remain those branches of knowledge which pertain not to the bodily senses, but to the intellect, among which the science of reasoning and that of number are the chief. The science of reasoning is of very great service in searching into and unravelling all sorts of questions that come up in Scripture, only in the use of it we must guard against the love of wrangling, and the childish vanity of entrapping an adversary. For there are many of what are called sophisms, inferences in reasoning that are false, and yet so close an imitation of the true, as to deceive not only dull people, but clever men too, when they are not on their guard. For example, one man lays before another with whom he is talking, the proposition, "What I am, you are not." The other assents, for the proposition is in part true, the one man being cunning and the other simple. Then the first speaker adds: "I am a man;" and when the other has given his assent to this also, the first draws his conclusion: "Then you are not a man." Now of this sort of ensnaring arguments, Scripture, as I judge, expresses detestation in that place where it is said, "There is one that showeth wisdom in words, and is hated;" [1819] although, indeed, a style of speech which is not intended to entrap, but only aims at verbal ornamentation more than is consistent with seriousness of purpose, is also called sophistical.
49. There are also valid processes of reasoning which lead to false conclusions, by following out to its logical consequences the error of the man with whom one is arguing; and these conclusions are sometimes drawn by a good and learned man, with the object of making the person from whose error these consequences result, feel ashamed of them and of thus leading him to give up his error when he finds that if he wishes to retain his old opinion, he must of necessity also hold other opinions which he condemns. For example, the apostle did not draw true conclusions when he said, "Then is Christ not risen," and again, "Then is our preaching vain, and your faith is also vain;" [1820] and further on drew other inferences which are all utterly false; for Christ has risen, the preaching of those who declared this fact was not in vain, nor was their faith in vain who had believed it. But all these false inferences followed legitimately from the opinion of those who said that there is no resurrection of the dead. These inferences, then, being repudiated as false, it follows that since they would be true if the dead rise not, there will be a resurrection of the dead. As, then, valid conclusions may be drawn not only from true but from false propositions, the laws of valid reasoning may easily be learnt in the schools, outside the pale of the Church. But the truth of propositions must be inquired into in the sacred books of the Church.
[1819] Qui sophistice loquitur, odibilis est. Ecclus. xxxvii. 20.
[1820] 1 Cor. xv. 13, 14.
Chapter 32.--Valid Logical Sequence is Not Devised But Only Observed by Man.
50. And yet the validity of logical sequences is not a thing devised by men, but is observed and noted by them that they may be able to learn and teach it; for it exists eternally in the reason of things, and has its origin with God. For as the man who narrates the order of events does not himself create that order; and as he who describes the situations of places, or the natures of animals, or roots, or minerals, does not describe arrangements of man; and as he who points out the stars and their movements does not point out anything that he himself or any other man has ordained;--in the same way, he who says, "When the consequent is false, the antecedent must also be false," says what is most true; but he does not himself make it so, he only points out that it is so. And it is upon this rule that the reasoning I have quoted from the Apostle Paul proceeds. For the antecedent is, "There is no resurrection of the dead,"--the position taken up by those whose error the apostle wished to overthrow. Next, from this antecedent, the assertion, viz., that there is no resurrection of the dead, the necessary consequence is, "Then Christ is not risen." But this consequence is false, for Christ has risen; therefore the antecedent is also false. But the antecedent is, that there is no resurrection of the dead. We conclude, therefore, that there is a resurrection of the dead. Now all this is briefly expressed thus: If there is no resurrection of the dead, then is Christ not risen; but Christ is risen, therefore there is a resurrection of the dead. This rule, then, that when the consequent is removed, the antecedent must also be removed, is not made by man, but only pointed out by him. And this rule has reference to the validity of the reasoning, not to the truth of the statements.