Compared with the other fields of mental life, the field of intellectual operations has given rise to a relatively limited num­ ber of experimental studies. And yet, in so far as all behaviour simultaneously comprises cognitive and affective aspects, one would think that the operations which alone make it possible to distinguish the true from the false would have attracted the attention of researchers at every phase of scientific psychology. T he fact that this has not been the case is probably due to two reasons. There is also a third reason which is fundamental.