Certain beliefs or judgements are indisputably irrational. When a victim of the Capgras delusion thinks that her husband has been replaced by an impostor, her mental state is definitely irrational. But irrationality is not confined to mental illness. Self-deception, wishful thinking, and denial are widespread, non-pathological cognitive phenomena that are also irrational. People in perfect mental health deceive themselves about their chances of winning the lottery, the intellectual talents of their kids, the fidelity of their husbands/wives, the probability that they receive a salary increase, etc. In short, mentally healthy people hold irrational beliefs in diverse kinds of circumstance. Do these various irrational beliefs have something in common? Is there anything that makes them all irrational? As its name suggests, the general purpose of the project “Irrationality” is to answer these questions and to offer, thereby, a philosophical account of cognitive irrationality.