Figure 17, when shown to a person, is perceived as the result of a child’s careless handling of his ink bottle, as an ink spot. But ask this person if he does not see a boy falling downstairs, and immediately certain elements are grouped together and affect us as being the legs, other elements of sensation are perceived as the arms, and so on. And now suggest to the same person to turn the page slightly to the right and see a man trying to put on his shirt. Quickly the perception changes again; but this time not so much by the breaking up of the former units into their sensory elements and the formation of new units, as by a change of the accompanying ideas. The previous suggestion tends to make us perceive these sensations in one or the other way because it guides our attention. But this guidance is possible only because certain groups of sensational elements (for example, the groups illustrated by our figures) have very often occurred in our mind in consequence of the fact that they originate from external objects which have often been presented to our sense organs among greatly varying surroundings. Thus we have learned to group these elements together and to neglect, more or less, all other elements which may be presented simultaneously. The total process of selective grouping and of furnishing the groups formed with additional mental contents has often been called _apperception_. But this meaning of the term apperception is not universally adopted. Some mean by apperception mainly the selective grouping of the elements, others mean by it exclusively the furnishing with ideational contents. Because of its ambiguity the term _apperception_ has been entirely omitted from the present book, and the term _perception_ is used in its broadest sense, including both the processes just mentioned. Perception thus means the working over by the mind of any aggregate of sensational elements given at the time through the sense organs. 2. _Illusions_ While the laws of perception are, on the whole, of the greatest benefit to the organism surrounded by a confusing multitude of physical elements bound together into a large number of more or less stable compounds, of things, there are exceptional cases in which these same laws lead the mind into a reaction not suitable to the situation presented.