The second kind of definition is, we have said, that by enumeration. It consists in placing before the eyes of the reader an assortment of psychological phenomena and then saying: "These are the things psychology studies." One will take readily as samples the ideas, reasonings, emotions, and other manifestations of mental life. If this is only a strictly provisional definition, a simple introduction to the subject, we accept it literally. It may serve to give us a first impression of things, and to refresh the memories of those who, by a rather extraordinary chance, would not doubt that psychology studies our thoughts. But whatever may be the number of these deeply ignorant persons, they constitute, I think, a negligible quantity; and, after these preliminaries, we must come to a real definition and not juggle with the problem, which consists in indicating in what the spiritual is distinguished from the material. Let us leave on one side, therefore, the definitions by enumeration. Now comes the definition by method. Numbers of authors have supposed that it is by its method that psychology is distinguished from the other sciences. To the mind is attached the idea of the within, to nature the idea of being without the mind, of constituting a "without" (_un dehors_). It is a vague idea, but becomes precise in a good many metaphors, and has given rise to several forms of speech. Since the days of Locke, we have always spoken of the internal life of the mind as contrasted with the external life, of subjective reality as contrasted with objective reality; and in the same way we oppose the external senses to the inner sense (the internal perception), which it has at times been proposed to erect into a sixth sense. Though no longer quite the Cartesian dualism, this is still a dualism. It has also been said that psychology is the science of introspection, and, in addition, that scientific psychology is a controlled introspection. This science of the "internal facts of man" would thus be distinguished from the other natural sciences which are formed by the use of our outer senses, by external observation--that is to say, to use a neologism, by externospection. This verbal symmetry may satisfy for a moment minds given to words, but on reflection it is perceived that the distinction between introspection and externospection does not correspond to a fundamental and constant difference in the nature of things or in the processes of cognition. I acknowledge it with some regret, and thus place myself in contradiction with myself; for I for a long time believed, and have even said in print, that psychology is the science of introspection. My error arose from my having made too many analyses of detail, and not having mounted to a sufficiently wide-reaching conception.