II. The two courses that we may pursue are (1) the direct, and (2) the
indirect refutation. The direct attacks the reason for the thesis; the
indirect, its results. The direct refutation shows that the thesis is
not true; the indirect, that it cannot be true.

The direct course admits of a twofold procedure. Either we may
show that the reasons for the statement are false (_nego majorem,
minorem_); or we may admit the reasons or premisses, but show that the
statement does not follow from them (_nego consequentiam)_; that is,
we attack the conclusion or form of the syllogism.

The direct refutation makes use either of the _diversion_ or of the
_instance_.

_(a)_ The _diversion_.--We accept our opponent's proposition as true,
and then show what follows from it when we bring it into connection
with some other proposition acknowledged to be true. We use the two
propositions as the premisses of a syllogism giving a conclusion which
is manifestly false, as contradicting either the nature of things,[1]
or other statements of our opponent himself; that is to say, the
conclusion is false either _ad rem_ or _ad hominem_.[2] Consequently,
our opponent's proposition must have been false; for, while true
premisses can give only a true conclusion, false premisses need not
always give a false one.

[Footnote 1: If it is in direct contradiction with a perfectly
undoubted, truth, we have reduced our opponent's position _ad
absurdum_.]