example, should he defend suicide, you may at
once exclaim, "Why don't you hang yourself?" Should he maintain that
Berlin is an unpleasant place to live in, you may say, "Why don't you
leave by the first train?" Some such claptrap is always possible.

[Footnote 1: The truth from which I draw my proof may he either (1) of
an objective and universally valid character; in that case my proof is
veracious, _secundum veritatem_; and it is such proof alone that has
any genuine validity. Or (2) it may be valid only for the person to
whom I wish to prove my proposition, and with whom I am disputing. He
has, that is to say, either taken up some position once for all as a
prejudice, or hastily admitted it in the course of the dispute; and
on this I ground my proof. In that case, it is a proof valid only for
this particular man, _ad kominem. I_ compel my opponent to grant
my proposition, but I fail to establish it as a truth of universal
validity. My proof avails for my opponent alone, but for no one else.
For example, if my opponent is a devotee of Kant's, and I ground my
proof on some utterance of that philosopher, it is a proof which in
itself is only _ad hominem_. If he is a Mohammedan, I may prove my
point by reference to a passage in the Koran, and that is sufficient
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 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.