Learning: change in organism"s behavior or thought as result of experience. Association: certain events occur together and one predicts the other. Interested in role of salivation in digestive process. Natural reflex: automatic association between stimulus and response. Learned reflex: new association learned where none existed before. Neutral stimulus (ns): control stimulus that shows no learned reflex. Unconditioned stimulus (ucs or us): unlearned / reflexive. Unconditioned response (ucr or ur): unlearned / reflexive reaction by the ucs. Conditioned stimulus (cs): previously neutral stimulus becomes conditioned through association with ucs. Conditioned response (cr): reaction to the cs. Rang bell to warn subjects on knee strike. Knee would jerk in response to bell even when knee not struck. Link ns and us so ns triggers cr. Occurs in classical conditioning when us does not follow cs. Reappearance after a pause of extinguished cr. Tendency, once response conditioned, for stimuli similar to cs to elicit similar.