The principle of _substitute stimulus_ applies remarkably well to practised perception. The first time you perceive an object, you observe it attentively, and expose your perceptive apparatus to the whole collection of stimuli that the object sends your way. The next time you need not observe it so attentively, for you make the same perceptive response to a _part_ of the original collection of stimuli. The response originally aroused by the whole collection of stimuli is later aroused by a fraction of this collection. The stimulus may be _reduced_ considerably, and still arouse the perception of the same fact. A child is making the acquaintance of the dog. The dog barks, and the child watches the performance. He not only sees the dog, and hears the noise, but he _sees_ the dog _bark_, and _hears_ the dog _bark_. This original perception is a unitary response to the combination of sight and sound. Thereafter he does not require both stimuli at once, but, when he hears this noise, he perceives the dog barking, and when he sees the dog he sees an object that can bark. In the same way, a thousand objects which furnish stimuli to more than one of the senses are perceived as units, and, later, need only act on a single sense to be known. The stimulus, instead of being reduced, may be _modified_, and still arouse the same perception as before. A face appears in the baby's field of view, but away across the room so that it is a very small object, visually. The face approaches and gradually becomes a larger visual object, and the light and shadow upon it change from moment to moment, but it remains nearly enough the same to arouse essentially the same perception in the child. He comes to know the face at various distances and angles and under various lights. {435} Again, the child holds a block in his hands, and looks at it square on, so that it is really a rectangle in his field of view. He turns it slightly, and now it is no longer visually a rectangle, but an oblique parallelogram. But the change is not enough to abolish the first perception; he sees it as the same object as before. By dint of many such experiences, we see a book cover or a door as a rectangle, no matter at what angle we may view it, and we know a circle for a circle even though at most angles it is really an ellipse in the field of view. A large share of practised perceptions belong under the head of "response by analogy",[Footnote: See p. 406.] since they consist in making the same response to the present stimulus that has previously been made to a similar but not identical stimulus. If every modified stimulus gave a new and different perception, it would be a slow job getting acquainted with the world. A thing is never twice the same, as a collection of stimuli, and yet, within wide limits, it is always perceived as the same thing. Corrected Perception