ABSTRACT
Our loyalty to our extant beliefs, warranted and unwarranted, is commonly reinforced by various belief-bias effects of which we are hardly ever aware: We tend to notice evidence that confi rms our beliefs more readily than uncongenial evidence, attach more weight to the former than to the latter, and accept argument more readily when it leads to a conclusion we believe true. In deductive reasoning, people are signifi cantly more likely to accept unsound arguments when they lead to conclusions they already believe true.1 They are also prone to unwittingly presuppose assumptions whose truth they take for granted. These two tendencies come together when we argue for a conclusion whose truth we take for granted all along: We may then presuppose the claim in the course of the very argument meant to support it and accept the argument thus rendered unsound.
