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, learns to discriminate against caterpillars. In a practical sense, the chick, like the rat, learns to distinguish between stimuli that at first aroused the same response. It is in the same way that the human being is driven to discriminate and attend to details. He is brought to a halt by the poor results of his first rough and ready perception, scans the situation, isolates some detail and, finding response to this detail to bring satisfactory results, substitutes response to this {437} detail for his first undiscriminating response to the whole object.