In this book (published in 2014 as part of the Key Concepts series at Polity Press) I discuss different conceptions of irrationality and review contemporary debates in philosophy and psychology that involve those conceptions.

Judgements of irrationality are made constantly and on many occasions they play an important role in our mutual interactions and social practices. This book is an attempt to understand what we take irrationality to be, what grounds our judgements of irrationality, and how irrationality affects human agency. Judgements of irrationality typically express disapproval towards behaviour deviating from a standard or violating a norm. But which standards and which norms? 

We talk about an agent being irrational or behaving irrationally when her behaviour defies our expectations and becomes hard to explain or predict; when her decisions are driven by emotions or instinct rather than by reflective deliberation; when her reasoning fails to conform to basic principles of logic and probability; when her beliefs are badly supported by evidence and conflict with the science of the day; when her actions and thoughts are self-defeating and undermine her wellbeing. The list is not exhaustive and we could go on. Depending on the context, agents exhibiting irrational behaviour may be regarded as foolish, ignorant, unwise, or even mad. The different conceptions of irrationality at play are often conflated, and one of the goals of the book is to examine distinct conceptions of irrationality and highlight elements of continuity and discontinuity among them.

The conceptions of irrationality I chose to investigate here are not entirely distinct and there are overlapping concerns in the debates I review. This means that some themes run throughout the book, such as the role of cognitive limitations in shaping human agency, the impact of irrational beliefs and emotions in mental wellbeing and good functioning, and the potential conflict between believing what is true and believing what is most conducive to happiness and success. All the themes above are approached with the methodology of empirically informed philosophical investigation.

The book debunks some philosophical myths about human rationality, and aims to contribute to a more psychologically realistic, but not entirely pessimistic, account of human agency.