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

Response by analogy, however, often leads us astray, in making us
perceive a new object as essentially the same as something already
familiar. First impressions of a new object or acquaintance often need
revision, because they do not work well. They do not work well because
they are rough and ready, taking the object in the lump, with scant
attention to details which may prove to be important. It is easy to
follow the law of combination and respond to a whole collection of
stimuli, but to break up the collection and isolate out of it a
smaller collection to respond to--that is something we will not do
unless forced to it. Isolation and discrimination are uphill work.
When they occur, it is {436} because the rough and ready response has
proved unsatisfactory,

_Substitute response_ is the big factor in corrected perception, as
substitute stimulus is in practised perception. When our first
perception of an object gets us into difficulties, then we are forced
to attend more closely and find something in the object that can serve
as the stimulus to a better response. This is the process by which we
isolate, analyze, discriminate.

Our old friend, the white rat, learned to enter a door only if it bore
a yellow sign. [Footnote: See p. 304.] It was uphill work for him,
hundreds of trials being required before the discriminating response
was established; but he learned it finally. At the outset, a door was
a door to the rat, and responded to as such, without regard to the
sign. Whenever he entered a door without the sign, he got a shock, and
scurried back; and before venturing again he looked all around,
seeking, we may say, a stimulus to guide him; incidentally, he looked
at the yellow disk, and this stimulus, though inconspicuous and feeble
to a rat, finally got linked up with the entering response. The
response of first finding and then following the sign had been
substituted for the original response of simply entering.

In the same way the newly hatched chick, which at first pecks at all
small objects, caterpillars included