INTRODUCTION

The present work is largely about irrationality. Yet the discussion will hardly make sense without a prior analysis of the notion of rationality. This is embarrassingly rich. There is a bewildering multitude of entities that are said to be rational or irrational: beliefs, preferences, choices or decisions, actions, behavioural patterns, persons, even collectivities and institutions. Also, the connotations of the term ‘rational’ range from the formal notions of efficiency and consistency to the substantive notions of autonomy or self-determination. And in the background of the notion lurks the formidable pair of ‘Verstand’ vs. ‘Vernunft’, be it in the Kantian or in the Hegelian senses.

I begin with the focus on rationality as a formal feature of individual actions (1.2). This will provide what, following a similar terminology in Rawls, I shall call the thin theory of rationality. It is thin in that it leaves unexamined the beliefs and the desires that form the reasons for the action whose rationality we are assessing, with the exception that they are stipulated not to be logically inconsistent. Consistency, in fact, is what rationality in the thin sense is all about: consistency within the belief system; consistency within the system of desires; and consistency between beliefs and desires on the one hand and the action for which they are reasons on the other hand.

The broad theory of individual rationality goes beyond these formal requirements (1.3).