Well, when Pavlov, for instance, started before conditioning there was simply an unconditioned stimulus, the food in the mouth, and an unconditioned response, saliva.
比如，在条件作用形成之前，只是简单的存在着无条件刺激，即食物，以及无条件反应，即唾液。
2
A type of learning in which a conditioned stimulus is paired with an unconditioned stimulus to elicit a reflex response or respondent behaviour - Also referred to as Pavlovian Conditioning.
In Pavlov's conditioning experiments with dogs, the presentation of food was the unconditional stimulus because it evoked the natural response of salivation in preparation for eating.