The struggle between our original feeling for truth and opposing interests creates a sophistic "dialectic" in which there seem to be arguments for all sorts of incompatible philosophical statements. Given that philosophical truth is not intuitive, we can only defend our feeling for truth by engaging in sound philosophical argumentation. So these are lectures in logic as applied to philosophy; and what they strive for is a general theory of dialectical error, in which the typical and most frequent philosophical fallacies are dissected in the light of actually occurring examples.