There is no evidence that the cat reasons his way out of the cage. His behavior is impulsive, not deliberative. There is not even any evidence that the cat clearly observes how he gets out. If he made a clean-cut observation of the manner of escape, his time for escaping should thereupon take a sudden drop, instead of falling off gradually and irregularly from trial to trial, as it does fall off. Trial and error learning is learning by doing, and not by reasoning or observing. The cat learns to get out by getting out, not by seeing how to get out. Summary of Animal Learning Let us take account of stock at this point, before passing to human learning, and attempt to generalize what we have observed in animals of the process of learning. (1) _Elimination_ of a response, which means _detachment_ of a response from the stimulus that originally aroused it, occurs in three main cases: (a) Elimination occurs most quickly when the response brings actual _pain_; the animal makes the avoiding reaction to the pain and quickly comes to make this response to the place where the pain occurred; and thus the positive reaction to this place is eliminated. (b) Elimination occurs more gradually when the response, without resulting in actual pain, brings _failure_ or delay in reaching a goal towards which the animal is tending. The positive response of entering and exploring a blind alley grows weaker and weaker, till the blind alley is neglected altogether. (c) Elimination of a response also occurs, slowly, through _negative adaptation_ to a stimulus that is harmless and also useless. {311} (2) New _attachments_ or _linkages_ of stimulus and response occur in two forms, which are called "substitute stimulus" and "substitute response". [Footnote: The writer hopes that no confusion will be caused by his use of several words to express this same meaning. "Attachment of stimulus and response", "linkage of stimulus and response", "connection between stimulus and response", and "bond between stimulus and response", all mean exactly the same; but sometimes one and sometimes another seems to bring the meaning more vividly to mind.] (a) _Substitute stimulus_ refers to the case where the natural response is not itself modified, but becomes attached to another stimulus than the one that originally aroused it. This new linkage can sometimes be established by simply giving the original stimulus and the substitute stimulus at the same time, and doing so repeatedly, as in the conditioned reflex experiment.