In Chapter 9, the suggestion was made that two distinct types of thought might be involved in selection-task experiments. From early 'therapy' studies, it appeared that there was a remarkable degree of independence between the process of selecting cards and the verbal evaluations of possible selections offered by the subjects. Similarly, Wason and Evans (1975), and Evans and Wason (1976), found that verbal justifications of selections were rationalisations, rather than accurate explanations of selection performance. This led to a theory of dual thought processes which has broad implications, not only for the psychology of reasoning, but for cognitive theory in general. The application of the theory to reasoning data has been revised recently (Evans, 1980a; 1980b), but we will first consider the original Wason and Evans ( 1975) version.