Arthur Schopenhauer 
The Art of Controversy 

First of all, we must consider the essential nature of every dispute: what it is that really takes place in it.
Our opponent has stated a thesis, or we ourselves, it is all one. There are two modes of refuting it, and two courses that we may pursue.
I. The modes are (1) ad rem, (2) ad hominem or ex concessis. That is to say: We may show either that the proposition is not in accordance with the nature of things, i.e., with absolute, objective truth; or that it is inconsistent with other statements or admissions of our opponent, i.e., with truth as it appears to him. The latter mode of arguing a question produces only a relative conviction, and makes no difference whatever to the objective truth of the matter.
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.