5 Reasoning and irrationality
In this chapter we consider the challenge presented to common-sense belief
by psychological evidence of widespread human irrationality, which also
conXicts with the arguments of certain philosophers that widespread ir-
rationality is impossible. We argue that the philosophical constraints on
irrationality, such as they are, are weak. But we also insist that the
standards of rationality, against which human performance is to be
measured, should be suitably relativised to human cognitive powers and
abilities.
1 Introduction: the fragmentation of rationality
According to Aristotle, what distinguishes humankind is that we are
rational. Yet psychologists have bad news for us: we are not so rational
after all. They have found that subjects perform surprisingly poorly at
some fairly simple reasoning tests – the best known of which is the Wason
Selection Task (Wason, 1968 – see section 2 below). After repeated ex-
periments it can be predicted with conWdence that in certain situations a
majority of people will make irrational choices. Results of this kind have
prompted some psychologists to comment on the ‘bleak implications for
human rationality’ (Nisbett and Borgida, 1975; see also Kahneman and
Tversky, 1972). Philosophers sometimes tell a completely diVerent story,
according to which we are committed to assuming that people are rational,
perhaps evenperfectlyrational. There has, until recently, been almost a
disciplinary divide in attitudes about rationality, with psychologists
seemingly involved in a campaign of promoting pessimism about human
reason, while philosophers have been trying to give grounds for what may
sound like rosy optimism.
It would seem that one or other of these views about human rationality
must be seriously wrong. But of course ‘rationality’ is hardly a well-deWned
notion, so when we look more closely into the matter the disagreement
may turn out more apparent than real. Before considering the arguments
and the evidence, we should notice that our pre-theoretical understanding
105