Book excerpt: Religious zeal, suicide terrorism, passionate commitment to ideologies, and the results of various psychological tests are often cited to show that humans are fundamentally irrational. The author examines all such supposed examples of irrationality and argues that they are compatible with rationality. Rationality does not mean absence of error, but the possibility of correcting error in the light of criticism. In this sense, all human beliefs are rational: they are all vulnerable to being abandoned when shown to be faulty.