XVII.

If your opponent presses you with a counter-proof, you will often be able to save yourself by advancing some subtle distinction, which, it is true, had not previously occurred to you; that is, if the matter admits of a double application, or of being taken in any ambiguous sense.

XVIII.

If you observe that your opponent has taken up a line of argument which will end in your defeat, you must not allow him to carry it to its conclusion, but interrupt the course of the dispute in time, or break it off altogether, or lead him away from the subject, and bring him to others.  In short, you must effect the trick which will be noticed later on, the mutatio controversiae. (See sec. xxix.)

XIX.

Should your opponent expressly challenge you to produce any objection to some definite point in his argument, and you have nothing much to say, you must try to give the matter a general turn, and then talk against that.  If you are called upon to say why a particular physical hypothesis cannot be accepted, you may speak of the fallibility of human knowledge, and give various illustrations of it.

XX.

When you have elicited all your premisses, and your opponent has admitted them, you must refrain from asking him for the conclusion, but draw it at once for yourself; nay, even though one or other of the premisses should be lacking, you may take it as though it too had been admitted, and draw the conclusion.  This trick is an application of the fallacy non causae ut causae.

XXI.

When your opponent uses a merely superficial or sophistical argument and you see through it, you can, it is true, refute it by setting forth its captious and superficial character; but it is better to meet him with a counter-argument which is just as superficial and sophistical, and so dispose of him; for it is with victory that you are concerned, and not with truth.  If, for example, he adopts an argumentum ad hominem, it is sufficient to take the force out of it by a counter argumentum ad hominem or argumentum ex concessis; and, in general, instead of setting forth the true state of the case at equal length, it is shorter to take this course if it lies open to you.

XXII.

If your opponent requires you to admit something from which the point in dispute will immediately follow, you must refuse to do so, declaring that it is a petitio principii For he and the audience will regard a proposition which is near akin to the point in dispute as identical with it, and in this way you deprive him of his best argument.

XXIII.