We shall see in Section 26 that our information about mathematical thought, and about creative thought in particular, is very limited. Yet this information, deriving for the most part from introspection, must be interpreted and used with extreme caution, not only because it is offered to us by mathematicians who are not necessarily psychologists and who, furthermore, can hardly be considered as disinterested observers, but above all because the few truly creative mathematicians who have wished to give a more or less detailed account of their innermost experiences do not apparently manifest one single type of thought activity.