Associative learning didn’t hit its stride until the work of Ivan Pavlov, which spurred the subsequent rise of the behaviorist movement in psychology. Pavlov introduced the concept of classical conditioning as a modernized version of associative learning. For Pavlov, classical conditioning was in part an experimental paradigm for teaching animals to learn new associations between stimuli. The general method of learning was to pair an unconditioned stimulus (US) with a novel stimulus. An unconditioned stimulus is just a stimulus that instinctively, without training, provokes a response in an organism. Since this response is not itself learned, the response is referred to as an “unconditioned response” (UR). In Pavlov’s canonical experiment, the US was a meat powder, as the smell of meat automatically brought about salivation (UR) in his canine subjects. The US is then paired with a neutral stimulus, such as a bell. Over time, the contiguity between the US and the neutral stimulus causes the neutral stimulus to provoke the same response as the US. Once the bell starts to provoke salivation, the bell has become a “conditioned stimulus” (CS) and the salivating, when prompted by the bell alone, a “conditioned response” (CR). The associative learning here is learning to form new stimulus-response pairs between the bell and the  salivation.[8]