Let no one say that this is only “imaginary,” meaning by this word that there are in my mind both the objectively true impression and an incorrect image of something similar. Such is not the case. There is no duality of consciousness. There is one unitary experience. Only scientific reflection reveals the fact that this unitary experience has two sources, one in the external stimulation, the other in the central nervous excitation. The result of these sources, the percept, does not betray the doubleness of its origin any more than a stream at its mouth shows the doubleness of its sources. It is a universal property of perception to be determined not by sensory stimulation alone, although this is the primary factor, but also by images, by nervous dispositions. The more vivid such images, the greater is their influence--now and then their _deceptive_ influence--on our consciousness of the objectively existing. Suggestion is a name which has recently been accepted for such an influence. Illusion is another name for it, in case it is rather pronounced and ill adapted to the object. QUESTIONS 125. What kinds of mental states are called perceptions? 126. Illustrate the change of a percept into a mental state not worthy of the name, caused by a change of the situation which involves neither a subtraction nor an addition of stimuli. 127. What impressions become a part of the percept, and what impressions do not? 128. Show that a percept contains not only less, but also more than the sensations corresponding to the stimuli of the moment. 129. What can we learn about perception from the drawings of children? 130. Illustrate the perception of a thing whose parts appear spatially separate. (None of the illustrations in the text strictly answers this question.) 131. What changes occur when a rabbit’s head is perceived as a duck’s head? 132. Are illusions signs of mental abnormality? What are they? 133. What two classes of illusions are distinguished in the text? § 15. IDEATION The same laws which govern the supplementing of impressions by images, govern also the supplementing of images by other images. We refer to the appearance of images supplementing other images by the word _remembering_, or _ideation_.