for him; but here it is only a proof _ad hominem_,]


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 § 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.