f _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