The author points out how the power of visualization has become gradually inhibited in the course of mental development (in the individual and the race) in favour of words, which constitute a more delicate instrument for thought. This inhibition may be of the 'suppression' type or of the 'fusion' type, in Rivers's terminology. In dreams and in certain other conditions (e. g. after shock) the tendency to strong visualization escapes from inhibition. Thoughts are then expressed in visual terms, as Freud has pointed out in the Interpretation of Dreams. In the course of a discussion of a dream reported by Nicoll, the author seems to indicate that the existence of this mode of expression explains the whole nature of the dream, making further analysis unnecessary.