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It is a long tradition in Western philosophy to characterize humans as rational animals and to argue that rationality is one of the features that distinguishes them from other animals. It is not just Aristotle who describes the human as the rational animal in his Metaphysics (1984). In Discourse on the Method (1985), Descartes also characterizes humans in terms of their distinctive reason or understanding; non-human animals do not have reason at all. This trend has continued: Donald Davidson says that rationality distinguishes ‘between the infant and the snail on one hand, and the normal adult person on the other’ (Davidson 1982, 318).

This view, however, can be (and has been) challenged. One challenge is to deny the sharp contrast between human cognition, which is rational, and non-human cognition, which is not. Humans are rational, but so are non-human animals. For example, Hume writes in his A Treatise of Human Nature that ‘no truth appears to me more evident than that beasts are endowed with thought and reason as well as man’ (Hume 1739/2007, 118).

Another challenge, which is the focus of this chapter, is to deny optimism about human rationality. Non-human animals are not rational, but humans are not rational either. In the memorable beginning of his essay ‘An Outline of Intellectual Rubbish’, Russell wrote:

Man is a rational animal – so at least we have been told. Throughout a long life I have searched diligently for evidence in favour of this statement. So far, I have not had the good fortune to come across it, though I have searched in many countries spread over three continents. (Russell 1961, 45)

Russellian pessimism about human rationality is echoed in a particularly influential psychological research programme in the 20th century, the heuristics and biases research programme, led by Amos Tversky and Daniel Kahneman. The studies in this programme revealed ‘systematic and severe errors’ (Tversky & Kahneman 1974, 1124) in human reasoning, which seem to have a ‘bleak implication for human rationality’ (Nisbett & Borgida 1975, 935). These studies caused heated debates on human rationality, sometimes dubbed ‘rationality wars’ (Samuels, Stich, & Bishop 2002), both at the theoretical and conceptual level and at the empirical and experimental level. The main aim of this chapter is to examine the relevant psychological studies to see if they really do have bleak or pessimistic implications for human rationality.

We start by presenting a definition of rationality (Section 1.2) before turning to the relevant psychological studies, in particular the ones from the heuristics and biases programme, which reveal a systematic failure to reason according to the rules of logic, probability, and decision-making (Section 1.3). The results of these studies support a pessimistic view of human rationality (Section 1.4). However, objections to the pessimistic interpretation were raised by Gerd Gigerenzer and his ecological rationality research programme (Section 1.5). The objections from this programme are significant because they bring to the fore important ideas that enhance our understanding of human rationality/irrationality. However, we shall argue that these objections do not refute the pessimistic interpretation. Although Gigerenzer’s objections do not refute pessimism, they lead us to examine an important issue: whether the difference between optimism and pessimism about rationality hangs on different accounts of the aim of cognition (Section 1.6). In the end, we defend a moderate form of pessimism, according to which humans are not as rational as we might have thought, before appreciating the results of the psychological studies on reasoning.