Informed by what was learned in recent decades, this eye-opening book sketches a nuanced view of rationality, both human and non-human. It aims to address recent challenges and identify a direction for future research on rationality; investigate the relationship between rationality and mental disorders, such as delusion and depression; assess reasoning in artificial intelligence and nonhuman animals; reflect on ancient Chinese Philosophy and possible cultural differences in human psychology; and employ philosophical reflection, along with linguistic, probabilistic, and logical techniques.