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.]

[Footnote 2: Socrates, in Hippia Maj. et alias.]<